One level in a stack of levels. A rank that exists only because there are others above or below it.
Old French tire (a row, a rank, a series), of origin not securely traced further back; one level in a stack of levels has always existed only in relation to the ranks above and below it.
A place in an ordered stack, meaningful only in relation to what sits above and below it. Nothing is a tier by itself; the word names a position, not a property.