Literally, the term "pAncarAtra" means "five nights". Some definitions tie back to this meaning, and others key in on the word "pAnca". Based on an analysis of the definitions found in various texts, the five rAtra-s have been equated to a) five cosmic elements, b) five objects of senses, c) the five types of knowledge, or d) the five types of ignorance. And so, the pAncarAtra Sastra can be considered either as the means for the liberation of the soul from the five elements, or container of the five types of knowledge, or the dispeller of the five types of ignorance. Some definitions tell the story of five nights which is described below.
The following is from vihagendra samhitA (I.31-4) - "When the kr*ta yuga had just appeared, by the grace of KeSava, the following five viz. ananta (the serpent), GaruDa, vishvaksena, Brahman and Siva hear the Sastra in parts, as follows: in the first night (rAtra) ananta has his questions answered, in the second night GaruDa, etc. Each of them hears for himself the Religion of Faith in the form of a work on knowledge, yoga, construction, and conduct, each night consisting of a lakh of verses. Since the whole instruction thus has five lakhs of verses, it is called the pAncarAtra".
The name "pAncarAtra" can thus be associated with the "five nights" of instruction. The last sentence in the above para from the translation of vihagendra samhitA implies that the word "rAtra" relates to "one lakh". This could not be verified from the dictionary.
See also: /Other interpretations of the term Pancaratara.