One word when used as a noun or adjective: Max Scherzer recorded 11 strikeouts; Nolan Ryan was a strikeout machine.

Two words, not hyphenated, when used as a verb: Walter Johnson could strike out any batter he faced; Alfonso Soriano struck out swinging.

When context allows, K may be used on second reference as an informal term for pitcher strikeouts: Pedro Martinez finished with 14 K’s in seven innings.

Never use K’s when referring to batter strikeouts, e.g. “Pete Alonso led all National League batters in K’s last season.”