A hue with black added — a darker version of the same colour.
In strict colour-theory usage, a shade is produced by reducing lightness/value while holding hue constant, distinct from a tint (lighter) or a tone (greyer).
In everyday speech 'shade' is often used loosely to mean any colour variant ('what shade of green is that?') — the strict definition specifically means darker.
Navy is a shade of blue.
Knowing the strict meaning helps when reading paint or design terminology precisely, even though everyday usage is looser.