Vac means speech, and in the Veda Speech is a goddess.
This is not as strange as it may sound. The Vedic poets lived by the spoken word. Their hymns were composed aloud, learned by ear and handed on from mouth to mouth, and a sacrifice worked only if every word in it was said correctly. To these poets, speech was the most powerful thing in the world. They gave her a name and a hymn of her own.
That hymn, the hundred and twenty-fifth of the tenth book, is one of the most remarkable in the Rigveda, because Vac speaks it herself. She says that she walks with all the families of the gods and holds up Mitra and Varuna, Indra and Agni. She calls herself the Queen. She says that everyone who eats, sees, breathes or hears does so through her, though they do not know it. She bends the bow of Rudra. Her home is in the waters, and she reaches up to touch the sky.
Other hymns look at her from the human side. One, addressed to Brihaspati, tells how the first seers gave names to things and so sent out the earliest speech. They then tracked Vac by means of sacrifice and found her living inside the seers. It warns that she does not show herself to everyone. One person looks and does not see her, and to another she reveals herself completely. A hymn of riddles says that speech has four parts. Three of them are hidden, and people speak only the fourth.
The poets also call her a cow who gives milk. A verse asks where her best part has gone.
She has no myth of birth or battle in the Rigveda. The stories about her begin in the Brahmanas, where she is a restless figure who keeps leaving the gods and has to be won back.
I am the Queen, the gatherer of treasures. The one I love, I make mighty: I make him a seer, I make him wise.
From the Vedas to the Puranas
What became of Vac
Vac did not keep a separate name in later Hinduism. She merged into Saraswati. The Brahmanas already say that Saraswati is speech, and by the time of the epics and Puranas the goddess of the word and the goddess of the river are one figure, the Saraswati who is worshipped today. One of Saraswati's common names, Vagdevi, simply means the goddess Vac.
Before that happened, the Brahmanas told a cycle of stories in which Speech has a will of her own. In the Shatapatha and Aitareya Brahmanas and the Taittiriya Samhita she is sent to the Gandharvas as the price of Soma, and chooses to return to the gods because they sing to her. In the Taittiriya Samhita and the Panchavimsha Brahmana she walks out on the gods and hides in the waters and the trees, which is why the drum, the lute and the flute have voices. In another passage of the Shatapatha she is refused as a priest's fee, turns into a lioness, and comes back only when the gods promise that she will be fed before the fire.
The same Brahmana tells of a quarrel between Speech and Mind over which of them is the greater. Prajapati judges for Mind, because Speech only repeats what Mind has thought. She tells him that she will never carry his offerings, and that is why rites for Prajapati are performed in a low voice.
The older picture never quite vanished. Her hymn, the Devi Sukta, is also found in the Atharvaveda, and it is chanted to this day in the worship of the Goddess. In Kashmir the goddess Sharada is praised in three forms, as Sharada, Saraswati and Vagdevi. Every time a child's first letters are written before Saraswati, it is still Speech who is being asked to come.