coalesce

Coalesce is a verb meaning to come together to form one mass or whole; to blend or fuse. It is used to describe the gradual merging of separate elements into a single entity in natural, social, and technical contexts. The term often implies a process of growth or union rather than abrupt change. In everyday language, rivers or streams can coalesce downstream, and small groups may coalesce into a single organization or committee.

The term derives from the Latin coalescere, from co- 'together' and alesce- 'to grow', meaning "to grow together." It entered English in the 16th century and retains the sense of unity through convergence.

In computing and data processing, coalescing has a specialized meaning. The COALESCE function in SQL returns the first non-null value from a list of expressions, providing a default when data may be missing. Its syntax is typically COALESCE(expr1, expr2, ...). If all arguments are null, the result is null. Many SQL dialects also offer equivalent constructs such as NVL or ISNULL, and various languages provide a null-coalescing operator.

In memory management, coalescing refers to merging adjacent free memory blocks into a larger block to reduce fragmentation and improve allocator efficiency. This technique is common in heap managers and garbage collectors, and involves a trade-off between additional bookkeeping and better long-term performance.