Definition of "tier"

tier¹

/ˈtaɪ.ə(ɹ)/

noun
plural tiers
1. One who ties (knots, etc).
2. Something that ties.
3. (archaic) A child's apron.

tier²

/ˈtɪə/

noun
plural tiers
1. A layer or rank.
  Quotations
  Turians have 27 citizenship tiers, beginning with civilians (client races and children). The initial period of military service is the second tier. Formal citizenship is conferred at the third tier, after boot camp. For client races, citizenship is granted after the individual musters out. Higher-ranked citizens are expected to lead and protect subordinates. Lower-ranking citizens are expected to obey and support superiors. Promotion to another tier of citizenship is based on the personal assessments of one's superiors and co-rankers.
  2008, BioWare, Mass Effect, Redwood City: Electronic Arts, PC, scene: Turians: Government Codex entry
  At various points during the (too frequent, mostly boring, school play-tier) dialogue.
  2023 May 25, Nic Reuben, “The Lord of the Rings: Gollum review”, in The Guardian‎
verb
third-person singular simple present tiers, present participle tiering, simple past and past participle tiered
1. (transitive) To arrange in layers.
2. (transitive) To cascade in an overlapping sequence.
3. (transitive, computing) To move (data) from one storage medium to another as an optimization, based on how frequently it is accessed.