Reusable BEX definition containing constants and functions that can be referenced by Compute steps. A Compute Definition lets a document keep shared deterministic logic in one place while still representing that logic as Blue data that affects document identity. Constants are literal values accessed with `$const`. Functions are named BEX programs with optional static Blue argument patterns. Definitions do not execute by themselves; a Compute step selects an entry function or imports the definition's functions/constants into its compilation context. Because this node is part of identity, changing a function body, argument pattern, constant, or description changes the definition's BlueId and therefore changes the semantic identity of documents that reference it.