Relating to or involving expression; of or pertaining to the expression of ideas or emotions.
From 'expression' plus the adjective suffix '-al'. Ultimately from Latin 'exprimere' (to press out).
While 'expressional' is technically valid, people almost always say 'expressive' instead—showing how English prefers certain word forms over others even when alternatives exist. This preference shapes which words survive.
Complete word intelligence in one call. Free tier — 50 lookups/day.