Possibly from Greek 'therizein' meaning 'to harvest,' or from the island Therasia. Famous via Saint Teresa of Ávila.
Teresa is one of the great traveling names of Christian Europe: warm in sound, but ancient and layered in origin. It is generally treated as a form of Theresa, a name whose exact etymology is still debated. Many scholars connect it with the Greek island name Thera or with a Greek word suggesting summer or harvest, though the trail is old enough that certainty has thinned.
What is clear is that Teresa became firmly established in the Romance languages, especially Spanish, Portuguese, and Italian, where it took on a graceful, devotional character. Its history is inseparable from two immense Catholic figures: Saint Teresa of Ávila, the 16th century Spanish mystic and reformer whose prose made her a literary as well as religious presence, and Mother Teresa of Calcutta, whose 20th century fame made the name recognizable around the world. Over time, Teresa has moved through several shades of style.
In medieval and early modern Europe it felt pious and serious; in the mid 20th century it became widely used in English-speaking countries and sounded familiar, gentle, and solid. Today it often reads as classic rather than fashionable, with an old-world dignity. The name also carries a rich literary and cultural afterlife, appearing in novels, songs, and devotional writing as shorthand for tenderness joined to strength.