Truism, n.
Etymology: [From True.]
An undoubted or self-evident truth; a statement which is pliantly true; a proposition needing no proof or argument; -- opposed to falsism. Trifling truisms clothed in great, swelling words. J. P. Smith.
Shades of meaning
Fernald has no entry of its own for truism; he lists it under proverb.
He lists it without comment; the entries above explain its neighbors.