Interior monologue
Direct rendering of a character's stream of thought, in its own syntax, without narratorial mediation.
Interior monologue renders a character's thought as it unfolds: associations, breaks, floating syntax, with no quotation marks or "he thought." Pushed to the extreme — punctuation abolished, pure associative logic — it becomes the stream of consciousness of the last chapter of Joyce's Ulysses.
Between classic reported thought and that radical flux lies a whole range, of which free indirect discourse is the most supple middle ground. Interior monologue excels at obsession, insomnia, intimate bad faith; its risk is indulgence — a stream of consciousness without dramatic necessity quickly becomes an open tap.
Example
"Too late now too late should have said it yesterday why yesterday already…"