means The formal act of beginning something or admitting someone into a group, often through a ceremony, ordeal, or rite of passage.

from From Latin 'initiare,' to begin or admit into the mysteries, built on 'initium,' a beginning — itself from 'inire,' to go in ('in-' plus 'ire,' to go). So at its root the word is simply about stepping inside; the Romans used it especially for entry into secret religious rites, which is why the word still carries that hush of thresholds crossed.