Upavita, Janivaara, Janiv, Or Poonal

YajnOpavita has several names but this is the most authentic Sanskrit name to it, and it clears states itself - the sacred thread to perform the vedic rituals to please Lord VishNu - the yajna or yajamAna.

In taitareeya aaraNyaka, 2nd prashna 1st anuvaaka supposedly has this anecdote:
"Once the devataas (gods) and the asuraas (demons) were fighting over who has the first right to the heavens. Each wanted to be the privilege, and pursued the yajna. The asuraas through their physical strengths very quickly accumulated all the resources needed for the yajna and started off. In their rush they did not follow the process prescribed in the vedas. However, the devas followed all the rules of brahmacharya and vedas. The asuras not knowing what is to be done and what is not to be, did not attain the heavens. The devas attained the heavenly planets by being diligent and sincerity. Because of their virtues the devas defeated the asuraas. It was the yajnopaveeta that the devas wore that served them, saved them, and yielded the positive results. Alas, the rites performed by the asuras lacking the yajnopavita did not yield any results. Therefore one should perform all rites upon wearing the yajnopaveeta"

यज्ञस्य प्रसृत्या अजिनं वासो वा दक्षिणत उपवीय दक्षिणं बाहुमुद्धरतॆऽवधत्तॆसव्यमिति  यज्ञॊपवीतं येतदॆव विपरीतं प्राचीनवीतगं संवितं मानुषम्
yajnopaveeta: When one wears the sacred thread from the top of the left shoulder down to below the right shoulders then it is called yajnopaveeta.
praacheenaaveeta: when one wears the sacred thread from the top of the right shoulders down to below the left shoulders
niveeta: the samveeta that is worn around the neck like a garland, usually during the shraaddhakaarya


निवीतं मनुष्याणां प्राचीनवीतं पितृणां उपवीतं दॆवानां

Vedas prescribe that when performing different karmaas one should wear the sacred thread in different ways, to indicate symbolically. When performing manuShyakaarya one should wear like the niveeta, during the pitrkaarya the praacheenaveeta, and during the dEvakaarya one should wear the upaveeta.

A symbolic anecdote to highlight the significance of the yajnopaveeta: 
Once Lord Brahma, the creator, manifested the three modes of material nature (satva, rajo, tamo) into three threads. He then manifested Himself into the three phases of life (shriti, sthithi, laya) into brahma, vishNu and maheshvara, and multiplied these together into nine threadlings. Since this was conceived by Lord Brahma, this has since been known as brahmasootra. Lord Brahma himself was the first to wear this.

The three layers of the brahmasootra indicates several aspects such as the three lOkas, three guNAs, three varNAs, three agnis etc. There is also an opinion that is worth sharing here. Before the yajnopaveeta is put together in its three layered form, it will be a three layered long threads that is approximately 96 inches in length (this is often measured based on the size of the finger). This is called षण्वति (ShaNvati, 96). According to varAha brahmopaniShat, this supposedly represents the 96 aspects or realities:

• 5 karmEndriyAs - vaak, vaaNi, pAda, pAyu, and upastha
• 5 panchpraaNAs - praaNa, apAna, vyAna, udAna, samAna
• 5 tanmAtrAs - shabdha, sparsha, roopa, rasa, gandha
• 5 panchabhootAs - pRuthvi, aap, tEjas, vAyu, aakaasha
• 6 bhAva-vikArAs - asti (being), jAyate (taking birth), vardhate, pariNamati, ksheeyate, naShyate
• 6 arishADvargAs - kAma, krodha, lObha, mOha, mada, maatsarya
• 6 shaDoormis - asana, pipAsa, shOka mOha, jarA, maraNa
• 6 dhAtus - tvak, rakta, mAmsa, mEdas, asthi, majja
• 4 antaHkaraNAs - manas, buddhi, chitta, ahaMkaara
• 3 avasthAs - jAgrat, swapna, suShupti
• 3 dEhAs -sthoola, sookShma, kAraNa
• 3 jeevAs - viShva, taijas, praajna
• 3 karmAs - prArabdha, saMchita, aagami
• 5 karmEndirya kriyAs - vachana, adAna, gamana, visarga, aananda
• 4 manah kriyAs - samkalpa, adhyavasAya, abhimAna, avadhAraNa
• 4 manas avasthA sthitis - mudita, karuNa, maitree, upEkShA
• 14 devatAs - dik, vAtaH, arkaH, prachEtAH, ashwini (2), vahni, indra, upEndra, mRutyu, chandra, chaturmukha, rudra, kshEtrajna
• 8 aatma guNAs - apahatapApma, vijaraH, vimRutyu, vishOka, vijighits, apipAsa, satyakAma, satyakankalpa
• 1 eeshwara or prabrahma 

So, the yajnopaveeta symbolizes and represents the above aspects of existence. The deities exist symbolically in the yajnopaveeta, i.e. everything is within reach for someone who puts on the yajnopaveeta with sincerity and dedication. As the popular saying goes "dharmo rakshati rakshitaH", i.e. it will protect those who protect it. The two ends of the ShaNvati are tied together, and if the two ends appear at the end it is supposedly called the "brahmagranthi" and if the two ends are put inside the knot then it is called to be "rudragranti". It is a common practice that the vaishNavaAs wear a brahmagranthi and the shaivaas wear a rudragranthi.

pooNal, yajnOpaveeta or janivAra
The Tamilians usually call the yajnopaveeta as poonal, where "poo"  means that which is sacred or pure or pavitra. "nool" means the gamut of vedic scriptures. "poonool" or "pooNal" then represents the sacred vedic scriptures. Alternatively, "nool" also means thread, as such it is the sacred thread. There is a vedic process behind how the multilayered threads are created via the chanting of the vedic mantrAs. There is a special way that the individual threads are brought together by tying the brahma or rudra granthi. Wearing this provides one the basic qualification of studying and understanding the srutis and smritis.

यज्ञात्मकः यज्ञस्वरूपिणः परमात्मनः तथा तद्भॊधकश्रुतिसृत्यादि

शास्त्राणां च उप समीपं वीयते नयति इति यज्ञोपवीतं  
That which enables one to comprehend the Lord and the scriptures which describe him, and that which takes or carries one closer to the shaastraas is called yajnOpaveeta.

In Kannada it is often referred to as "janivAra", which is infact a Sanskrit word. Because of it purifying nature, the yajnOpaveeta has the potency to enable one to transcend the platform of janma and mrutyu. It has the potency to free the chetana from aagaami, sanchita and praarabdha karmAs, and to liberate the soul from this material world. Because of its ability to free the individual from rebirth it is said "जनि-प्रादुर्भावं वारयति निवारयति इति जनिवारः". 

Wearing the yajnOpaveeta
A brahmachaari, along with the ajina wears one set of the yajnOpaveeta, the gRuhastha and vAnaprastha wear two upaveetAs, and the sanyAsi wears only one. Sage Devala says that a gRuhastha can wear three upaveetaas, that people from all the four ashramas should wear the yajnOpaveeta, and if one takes it away then he will have to perform austerities and the upanayana should be done again.

ब्रह्मसूत्र परित्यागात् ब्रह्मचारी गृही वनी । परिव्राच्चापि पतति तस्मात् तन्न परित्यजॆत् ।
त्यत्वा यज्ञोपवीतं तु ष्ट्कृच्छ्राणि समाचरॆत् । सावित्री सहितं कुर्त्यात् पुनरप्युपनायनम् ॥

If one stays without the yajnOpaveeta for a day then he will be relegated to the status of a shudra, and is said to have seen death (because one loses the birth he took by wearing it). Therefore all the four aashramAs should wear the yajnOpaveeta at all times.

विना यज्ञॊपवीतॆन दिनमॆकपि द्विजः । स्थितः शूद्रत्वमाप्नॊति मृतश्चैवॊपजायते । तस्मात् चतुर्भिराश्रमिभिः उपवीतं नित्यं धार्यं ॥

If one loses the yajnOpaveeta by accident (while removing a shirt or in a river etc), then he should wear one at the earliest, or atleast wear a cotton thread in place, and find the yajnOpaveeta as soon as possible.

The yajnOpaveeta dhaaraNa mantra signifies the value of the yajnOpaveeta:

ॐ यज्ञोपवीतम् परमं पवित्रं प्रजापतॆर्यथ्सहजं पुरस्तात् ।

आयुष्यमग्र्यं प्रतिमुंच शुभ्रं यज्ञोपवीतं बलमस्तु तॆजः ॥

Om! this yajnOpaveeta is the best among those that purity, that which is extremely sacred, that which was born along with Lord Brahma in the beginning; that which increases one's life and prominence, that which is sacred and clean, and confers both strengths and charm to the one who wears it. 

Reference: samskaara mahOdadhi, Dr. SwacChaMda anantanarasiMhAchAr