:"outing" usually translated as "adventure"
In Irish myth, an echtrae is an excursion to the Otherworld.
There has long been some confusion between the two genres of echtrai and immrama, due mainly to the mistitled Immram Brain, that is, The Voyage of Bran. I say it is misnamed, for the term immram--"rowing out"--is used to refer to sea voyages, particularly those of the monastic period, such as the Voyage of St. Brendan, the Voyage of Maelduin, or the Voyage of O'Curra. These are tales, although containing fantastic elements, which are not concerned with the otherworld; they are firmly set in this world, and specifically a Christianized world. In each immram, the hero is a Christian, and though he is faced with supernatural elements, he never changes his religion.
However, in The Voyage of Bran, we actually have an echtrae, not an immram. The echtrae is specifically concerned with the adventure of a hero in the Otherworld. It is firmly pagan in nature--gods and goddesses often beckon the hero on the voyage, which leads him to one of the many islands of the otherworld: Mag Mell, Tír na nÓg, Tír inna mBan, etc. The god is often Manannán mac Lir, and the goddesses one of his daughters, such as Niamh, and they are usually bearing a flowering branch of apples. The point of the echtrae is either to lure a hero to the Otherworld for eternity as a husband for one of the goddesses, such as the case of Connla or Oisín; or so as to impart some form of wisdom or power unto the hero, such as the case of Cormac mac Airt. Only the chosen return from the echtrai, and those who return against the will of the gods usually perish, having found that they had been away from Ireland for hundreds of years.
So what of the case of Immram Brain? Why is it called immram instead of echtrae? It was likely that Bran's story became confused with that of St. Brendan, and the term immram was passed on to Bran's story, despite its unsuited nature.
Ancient Irish Tales, ed. and trans. T. Cross & H. Slover. NY: Barnes and Noble, 1996 (reprint). This contains most of the echtrai in the Fionn Cycle and the Irish Cycle of the Kings
Dumville, David N. "Echtrae and Immram: Some problems of Definition." Ériu Vol. XXVII. Dublin: Royal Irish Academy, 1976. pp 73-94.