Irish form of Catherine, from Greek 'katharos' meaning pure.
Kathleen is the Anglicized form of Caitlin, the Irish form of Katherine or Catherine. That takes it back through medieval Europe to the Greek Aikaterine, though the deepest origin of that ancient name is still debated. Over the centuries, Catherine became associated with the Greek word katharos, meaning "pure," and that traditional meaning has long shaped the way Kathleen is understood.
The Irish route matters here: Kathleen is not just another Catherine-form, but a specifically Gaelic-flavored one, softened and musical in English. The name blossomed in the English-speaking world during the 19th and early 20th centuries, especially where Irish identity carried emotional and political resonance. The poem and song "Kathleen Mavourneen" helped romanticize it, making Kathleen seem wistful, lyrical, and unmistakably Irish to many listeners far beyond Ireland itself.
In America it became especially popular in the first half of the 20th century, often chosen by Catholic and Irish American families. Cultural bearers such as actress Kathleen Turner later gave it a more modern strength, but its deepest atmosphere remains classic and warm. Today Kathleen may feel less fashionable than it once did, yet that is part of its appeal: it has moved from peak popularity into the realm of inheritance. It sounds educated, gracious, and grounded, with a gentle old-song echo behind it.