head·er / ˈhedər/ • n. 1. Soccer a shot or pass made with the head. 2. inf. a headlong fall or dive. 3. a brick or stone laid at right angles to the face of a wall. Compare with stretcher (sense 4). 4. a line or block of text appearing at the top of each page of a book or document. Compare with footer (sense 2). 5. (also header tank) a raised tank of water maintaining pressure in a plumbing system. 6. a beam crossing and supporting the ends of joists, studs, or rafters.
The header of a data-structure representation is logically distinct from the data elements themselves and may serve several purposes:
(a) to hold global information about the whole structure, e.g. list length, array index bounds;
(b) to represent an empty structure;
(c) to provide links into the structure, e.g. pointers to first and last nodes in a list;
(d) to represent the entire structure in any other data structures of which it may be a part.
In networking, the header (or head) of a cell or packet is the part that holds routing and control information used to control the passage of the cell or packet across a network.