Rate-Determining Step
The slowest step in a reaction mechanism, which limits the overall rate.
Unit
Dimensionless
In depth
Because every later step must wait for it, the rate law of the overall reaction is governed by the rate-determining step together with any equilibria preceding it. Species that appear only after this step have no influence on the rate, which explains rate laws that omit a reactant entirely.
Examples in the real world
In SN1 substitution the slow carbocation formation controls the rate, so the nucleophile does not appear in the rate law.