An Echtra or Echtrae (pl. Echtrai) is one of a category of Old Irish literature about a hero's adventures in the Otherworld (see Tír na nÓg and Mag Mell); the otherworldly setting is the distinctive trait of these tales. More generally, echtra was the Old Irish word for "adventure", the Modern Irish word is eachtra.
The echtra was one of the most popular of Old Irish genres, so much so that the word later came to be used in the titles of any romance, regardless of otherworldly content. Earlier on, however, an echtra's emphasis was on the hero's time in the Otherworld, the journey to which served merely as a frame story. This distinguishes the echtrai from the Immrama, or "Voyages", which focus on the hero's journey rather than the otherworldly destination.
The hero of the echtra is usually invited to the Otherworld by a beautiful maiden or a great warrior, and he must cross either the western ocean or a plain blanketed by a mystical fog. The host is revealed to be one of the Tuatha Dé Danann, or fairy folk, and Manannan or Lugh often figure into the tale. The hero's fate after his sojourn varies from tale to tale. Sometimes he stays among the sídhe forever, and sometimes he returns with knowledge and gifts for his people. Sometimes the hero discovers his visit has lasted for years or even centuries though he thought no time had passed. He is warned that if he ever touches his home soil again, he will surely perish. In the Voyage of Bran, the heroes describe their adventure to listeners shore, then sail off into oblivion. In a popular story from the Fenian Cycle, Oisín touches the ground and instantly ages three hundred years. He tells his story to Saint Patrick and received a Christian baptism before he died.