A base colour plus the two colours next to its complementary opposite, rather than the exact opposite itself.
A base hue plus the two hues at roughly ±150° from it (i.e. adjacent to the 180° complementary point).
Split-complementary is often confused with a plain complementary pairing — the 'split' specifically means two near-opposite hues, not one exact opposite.
Blue paired with yellow-orange and red-orange (rather than plain orange) is a split-complementary scheme.
Split-complementary schemes give much of a complementary pairing's contrast with slightly less visual tension.