Pabbajjā: lit. ‘the going forth’, or more fully stated, ‘the going forth from home to the homeless life’ of a Bhikkhu agārasmā, anagāriyam consists in severing all family and social ties to live the pure life of a monk, in order to realize the goal of final deliverance pointed out by the Enlightened One. Thus, p. has become the name for admission as a sāmanera or novice, i.e. as a candidate for the Order of Bhikkhus, or Bhikkhus.

See Going Forth, by Sumana Samanera WHEEL 27/28 and
Ordination in Theravāda Buddhism WHEEL 56.