A secret held so that knowing it proves who you are. A password protects only while it stays unshared.
Latin passus (a step, a passing) + Old English word (speech); a 19th-century military term for the spoken word that earns passage through a guarded line, narrowed now to a typed secret.
A secret held so that knowing it stands in for who someone is. It protects a place only for as long as it stays unshared.