1. n. [Drilling]

The adapter between the first casing string and either the BOP stack (during drilling) or the wellhead (after completion). This adapter may be threaded or welded onto the casing, and may have a flanged or clamped connection to match the BOP stack or wellhead.

Synonyms: casinghead

See related terms: casing spool,  cellar,  flange,  surface casing