any rigid structural member projecting from a vertical support, esp. one in which the projection is great in relation to the depth, so that the upper part is in tension and the lower part in compression.
2.
Building Trades,Civil Engineering. any rigid construction extending horizontally well beyond its vertical support, used as a structural element of a bridge (cantilever bridge), building foundation, etc.
3.
Aeronautics. a form of wing construction in which no external bracing is used.
4.
Architecture. a bracket for supporting a balcony, cornice, etc.
cantilever (kān'tl-ē'vər, -ěv'ər) Pronunciation Key
A projecting structure, such as a beam, that is supported at one end and that carries a load at the other end or along its length. Cantilevers are important structures in the design of bridges and cranes.
[kan-tl-ee-ver, -ev-er] 
tlˌi