il·lus·tra·tion (ĭl'ə-strāʹshən)n. 1. a. The act of clarifying or explaining. b. The state of being clarified or explained. 2. Material used to clarify or explain. See Synonyms at example. 3. Visual matter used to clarify or decorate a text. 4.Obsolete. Illumination. il'lus·traʹtion·al adj.