In ahirbudhnya samhitA, at the end of the 11th adhyAya, it is stated that the Lord Himself framed out of the original Sastra the system called pAncarAtra describing His fivefold nature known as para, vyuha, vibhava, antaryAmin, and arca forms. This is yet another interpretation for the term.
Other explanations involve the five-fold kAla-s like abhigamana etc., or the panca samskAra-s (puNdra, nAma, mantra, japa and pujA). In the nArAyaNIya (12.338.4) nArAyaNa is called "pa~ncaj~na, pa~nca kAla kartr* pate, pa~ncarAtrika" - (Homage to) Thee of Five Offerings, Lord of those who perform the Pa~nca kAla ritual, Thee of pA~nca rAtra. In the pAdma-samhitA there is a Sloka which lists different words which are synonymous with the word 'bhAgavata'. One of these is panca-kAla-vit (the knower of the significance of the five-fold division of time). "sUrih suhr*t bhAgavatah sAttvatah panca-kAla-vit | ekAntikah tan-mayaSca pAnca-rAtrika ityapi | evamAdibhirAkhyAbhih Akhyeyah kamalAsana || (2-87)" SrI vedAnta-deSika's eloquent defense of pAnca-rAtra highlights this aspect of the pAnca-rAtra doctrine -
panca-kAla vyavasthityai venkateSa vipaScitA | SrI pAncarAtra siddhAnta vyavastheyam samarthitA || The jayAkhya samhitA gives a similar definition, and says the name is derived based on the five different actions performed during the five time periods - panca karmAnushThAna panca kAla nirdeSanena pAncarAtra iti sam~jnah. The panca kAla-s are: 1) abhigamana - beginning with the brahma-muhUrta about 4.30 a.m., devoted to the preparatory sequences in the morning (like Sauca, snAna, snadhyA, japa, and dhyAna).
2) upAdAna - from about 9 a.m. to 12 noon) - spent in collecting the articles necessary for worship (like flowers, tulasi, fruits, etc.), and making preparations for worship
3) ijyA ( beginning with noontime) devoted to the formal iconic worship elaborately conducted, and partaking of food as naivedya with other devotees
4) svAdhyAya (afternoon and evening), spent in the study of scriptures, contemplating on import, teaching scriptures to others, and such activities
5) yoga (night), devoted to the repetition of the formal iconic worship but briefly conducted, yoga practice (eight-fold practice, ashThAnga yoga), and deep meditation on godhead (dhyAna) before finally resting for the day.
The five phases are intended to be one uninterrupted worship ritual, with the ijyA as the central theme, and the others as preparatory or follow-up. This may be along the lines of the pAncarAtra sattra ritual of the Satapata-brAhmaNa, with the five nights altered to five phases of the day.
Among the definitions that do not directly relate to the practices of the pAncarAtra are the following:
The mahA-bhArata views is that the name derives from the fact that pAnca-rAtra is inclusive of the four veda-s and the sAnkhya yoga - "idam mahopanishadam caturveda samanvitam | sAnkhya yoga kr*tAntena panca-rAtrAnuSabditam " ( mahAbhA 339,112).
The five subjects of knowledge covered in the pAncarAtra according to the nArada pAncarAtra are tattva (cosmology), mukti-prada (liberation), bhakti-prada (devotion), yaugika (yoga), and vaishayika (objects of sense). The nAradIya pAncarAtra is now generally not considered an authentic pAncarAtra text by many vaishNava-s. I could also not verify the meaning "knowledge" for the word "rAtra" from my samskr*t dictionaries.
The Sandilya samhitA (1-4-75-76) gives a different interpretation viz. that the pAncarAtra is capable of giving the devotee delight that will be given together by the five nights viz. sAnkhya, yoga, Saiva, veda-s, and AraNyaka. Here the firve Sastra-s are equated with rAtra-s.
Another interesting stanza from the utsava sa~ngraha says that the name derives because of the five disciplines covered in the pAncarAtra - Agama, bharata (nAtya Sastra), Silpa (iconography and architecture), vaidya (medicine), and jyotisha (astrology) -
Agamam bharatam Silpam vaidya jyotishameva ca | pancA SAstrANi samyogAt pAncarAtramiti smr*tam || (utsava san. Mss. Adyar Lib., vo. 3 p. 15). Paushkara samhitA (38-307-308) interprets the word as meaning a collection of five systems of thought - purANa, veda, vedAnta, sAnkhya, and yoga
- purANam veda-vedAntam tathAnanAkhya yogajam | panca prakAram vij~neyam yatra rAtrAyate'bjaja | phalotkarshenaiva pancarAtra iti smr*tam ||
There are some texts (e.g., padma-purANa) that interpret the word rAtri as 'overcoming' or 'eradicating' (rAtrIyante, adr*SyAni bhavanti), some that interpret rAtra as meaning ignorance or false knowledge (e.g., vishNu samhitA), etc. The false knowledge is that obtained through the five senses (form, sound, smell, taste, and touch), or the panca-bhUta-s (earth, water, fire, air, AkASa) which constitute the world.