Bhujanga prayātam is a meter which has twelve syllables in each quarter (pāda). Only first, fourth, seventh and tenth are light. Rest of the syllables are heavy syllables.
In the Natya Shastra Chapter 16 which was written by Bharata muni, the definition is given (Quoting from wisdomlib.org).
- [The metre with] the feet of twelve syllables of which the first, the fourth, the seventh and tenth are light [and the rest heavy], is called Aprameyā.
What is a light syllable and a heavy syllable?
A light syllable means a syllable which takes a moment to spell. It is called a Laghu meaning short or light (denoted by I).
A heavy syllable means a syllable which takes two moments (or longer than usual) to spell. It is called a Guru means heavy (denoted by U).
There are rules on how to identify a guru and a laghu. A syllable becomes guru if
it has a long sound E.g : In the word रामायणम् , रा and मा are gurus.
it ends with a closing sound (called pollu in Telugu) E.g : In the word रामायणम्, णम् is counted as a single word and is identified as a guru.
the succeeding syllable is a samyuktakshara or a dvitvakshara. E.g : In the word निर्विकल्प, both नि and क are gurus as they are succeeded by samyuktaksharas. In the word विशुद्ध, शु becomes guru.
There are other rules too.
Identifying guru and laghu is called as Gaṇa vibhajana. Gaṇa as we is group. In this context, it is a group of two or more letters. E.g III is called a na gaṇa. IUU is a ya gaṇa.
This Bhujanga prayāta which is also known as Aprameyā is a type of vritta. There are several types of vrittas. From Wikipedia :
The Vritta stanzas are further recognized in three forms, with Samavritta where the four quarters are similar in its embedded mathematical pattern, Ardhasamavritta where alternate verses keep similar syllabic structure, and Vishamavritta where all four quarters are different. A regular Vritta is defined as that where the total number of syllables in each verse is less than or equal to 26 syllables, while irregulars contain more. When the metre is based on morae (matra), a short syllable is counted as one mora, and a long syllable is counted as two morae.
In other words, after one laghu, there are two gurus in each quarter (feet or pāda in Sanskrit). So, it has four 'ya' ganas. For example if we look at the first pāda of Vishnu Bhujangam composed by Adi Shankara (Quoting from Sanskritdocuments.org):
चिदंशं विभुं निर्मलं निर्विकल्पं
After identifying guru and laghus,
IUU IU UIU UIUU
In the above line, all are gurus except the first, fourth, seventh, tenth syllables. There are two gurus after a laghu. One I followed by U. So, there is a rythm of up and down while we read a poem or stotra written in this meter.
Why is this named as Bhujaṅga prayāta?
This meter is named as Bhujanga by ancient Indian mathematician Pingala (Source) because its composition resembles the serpentine movement of a snake. Prayātam means advancement or movement.
Many stotras are written in this meter. One of them which doesn't have a suffix bhujanga is Shivashtakam which starts with the lines
Prabhuṃ prāṇanāthaṃ vibhuṃ viśvanāthaṃ jagannāthanāthaṃ sadānandabhājam