S rank

Short definition from Wiktionary

Noun

1. 1
   An exceptional rank given on an objective, mission, or level in a game. Typically the highest achievable such rank.
2. 2
   A ranking above A in various situations, including tierlists.

Adjective

1. 1
   Of exceptional quality in its class.

Verb

1. 1
   To achieve an S rank.