A mast step is the structural foundation or socket, typically located on the keel or deck, that secures the base (heel) of a sailboat's mast. It supports the vertical compression load of the mast and transfers rigging forces to the hull, preventing the mast from moving laterally or longitudinally.

Key Aspects of the Mast Step:
Location: Keel-stepped masts (stronger, on the bottom of the hull) or deck-stepped masts (easier to install, on top of the deck).

Function: Holds the mast securely in place, often paired with a "mast partner" (deck support) to prevent movement.

"Stepping" the Mast: The, act of installing the mast into this fitting.

Types: Can be fixed, folding, or adjustable in some racing classes to control mast rake (forward/aft tilt).

Often, particularly in older traditional ships, a "mast step" is also where protective coins were ceremonially placed for good luck.