A crab boil is a one-pot method of cooking whole crab, often alongside other shellfish, by boiling it with potatoes, corn, and a heavy seasoning blend, then serving everything together, usually dumped straight onto a table. It's closely related to the Low Country boil (or Frogmore stew) of the South Carolina and Georgia coast, though that dish leans more on shrimp and sausage; a crab boil puts whole crab at the center.
The technique traces back to Gullah and Geechee cooking on the Sea Islands, where West African-influenced communities developed communal, one-pot boiling as an efficient way to feed large groups. Ingredients go into the pot in order of cooking time - potatoes and corn first, then the seafood - and the water is heavily seasoned, often with cayenne, paprika, mustard seed, or Old Bay.
It's traditionally served on a newspaper-covered table, eaten by hand, with crusty bread, cold beer, and lemon wedges. Regional versions swap in andouille sausage, clams, or mussels depending on what's local.