The word to designate Him (Isvara) is Pranava or OM.

Verse 27

तस्य वाचकः प्रणवः ॥२७॥

tasya vācakaḥ praṇavaḥ ॥27॥

The word to designate Him (Isvara) is Pranava or OM.

In the verse 27, Sage Patanjali is stressing on the Ishvara and the symbol, the mark of His, i.e. Aum. 

Symbol is the manifestor of the thing signified, and if the thing signified has already existence, and if, by experience, we know that the symbol has expressed that thing many times, then we are sure that there is the real relation between them. Even if the things are not present, there will be thousands who will know them by their symbols. There must be a natural connection between the symbol and the thing signified; then, when that symbol is pronounced, it recalled the thing signified. 

Patanjali says the manifesting word of God is Om. Why does he emphasize this? There are hundreds of words for God. One thought is connected with a thousand words; the idea, God, is connected with hundreds of words, and each one stands as a symbol for God. 

 

But there must be a generalization among all these words, some substratum, some common ground of all these symbols, and that symbol, which is the common symbol will be the best, and will really be the symbol of all. In making a sound we use the larynx, and the palate as a sounding board. Is there any material sound of which all other sounds must be manifestations, one which is the most natural sound? Om (Aum) is such a sound, the basis of all sounds. 

The first letter, A, is the root sound, the key, pronounced without touching any part of the tongue or palate; M represents the last sound in the series, being produced by the closed lip, and the U rolls from the very root to the end of the sounding board of the mouth. Thus, Om represents the whole phenomena of sound producing. As such, it must be the natural symbol, the matrix of all the variant sounds. It denotes the whole range and possibility of all the words that can be made. Apart from these speculations we see that around this word Om are centered all the different religious ideas in India; all the various religious ideas of the Vedas have gathered themselves round this word Om. 

PATANJALI YOGA SUTRAS