1. A unifying bond or link.
2. (anatomy) A band-like structure uniting two or more parts, similar to a ligament.
3. (mathematics) A horizontal line over two or more elements of a mathematical expression, to indicate they are to treated as a single term; this is equivalent to brackets or parentheses around the terms. For example, z - x̅+̅y̅ is the same as z - (x+y). Examples here.
4. The heart of a Borg ship.
Etymology: Latin, from vinciere, a tie or bond. In the sense of the first definition, it's been in use since the 17th century.
As an anatomy term, it was first used in the 19th century.
As a Borg term, the definition is more recent.
(As a mathematical symbol, the horizontal line was first used in the 12th century.)