Ahilyaa is a Raamaayan character. We meet her in Baal Kaand when Raam gives her Mukti from her husband Muni Gautam's Shaap, from a stone touching it with His feet.

Once Brahmaa Jee took the best from His creation and created a woman, and named her Ahilyaa. She was extremely beautiful. Now what to do with her? so He gave her to Rishi Gautam to keep until the decision is made. Ahilyaa started living with Rishi Gautam and served him sincerely. She lived there for a long time then Brahmaa Jee finally gave her to Rishi Gautam, thus she became Rishi Gautam's wife.

So Ahilyaa was the wife of Gautam Rishi and was a Pativrataa (loyal to husband) woman. They had a son named Shataanand (he was Raajaa Janak's Purohit). Once Raajaa of Devtaa, Indra, got attracted to her and had a desire to have her, but was afraid of her Paativrat Dharm. So one day when Gautam Rishi was out for his daily chores in early morning hours, He came in the form of Gautam Rishi and expressed His desire to have her. Ahilyaa was very surprised to see Rishi returning home and hear this strange type of proposal from Muni, but since she was a Pativrataa woman she considered to obey her husband. Thus He had her. 

When Indra was coming out of Rishi's Aashram, He met Gautam Rishi at the door. Both saw each other. Indra got frightened to see Rishi so He ran away. Muni understood everything, he came inside. Seeing Rishi Gautam again Ahalyaa also got frightened and understood everything. She fell on the feet of Gautam Muni and asked for his forgiveness as she was caught innocent. That was not her fault. But Muni was very angry and he gave Shaap to her, "Be a stone". Then later he relaxed it with "When Bhagvaan will take Avataar as Raam in Ayodhyaa in Tretaa Yug and will pass from here, you will again become the woman by the touch of His feet You will have your beauty back and you will come back to me."

So when Vishwaamitra Jee was taking Raam and Lakshman to Mithilaa, seeing a big stone on the way in an Aashram He asked as what type of stone it was? Vishwaamitra Jee told Him the story and asked Him to touch the stone with His foot. Raam touched the stone with His foot. The stone turned into a beautiful woman Ahilyaa. She prayed Raam and went to her husband's Lok.