Teresa of Ávila, OCD (born Teresa Sánchez de Cepeda y Ahumada; 28 March 1515 – 4 or 15 October 1582), also called Saint Teresa of Jesus, was a Spanish Carmelite nun and prominent Spanish mystic and religious reformer.

Active during the Counter-Reformation, Teresa became the central figure of a movement of spiritual and monastic renewal, reforming the Carmelite Orders of both women and men. The movement was later joined by the younger Spanish Carmelite friar and mystic John of the Cross, with whom she established the Discalced Carmelites. A formal papal decree adopting the split from the old order was issued in 1580.

Her autobiography, The Life of Teresa of Jesus, The Interior Castle, and The Way of Perfection, are prominent works on Christian mysticism and Christian meditation practice. In her autobiography, written as a defense of her ecstatic mystical experiences, she discerns four stages in the ascent of the soul to God: mental prayer and meditation; the prayer of quiet; absorption-in-God; ecstatic consciousness.

Forty years after her death, in 1622, Teresa was canonized by Pope Gregory XV. On 27 September 1970 Pope Paul VI proclaimed Teresa the first female Doctor of the Church in recognition of her centuries-long spiritual legacy to Catholicism.