Illustrated Architecture Dictionary
pen DEN tive
1. A concave triangle supporting a corner of a circular dome built over a square space.
A triangular curved surface between two arches and beneath a dome.
The concave triangular sections of vaulting that provide the transition between a dome and the square base on which it is set to transfer the weight of the dome.
2. A pendentive is a constructive device permitting the placing of a circular dome over a square room or an elliptical dome over a rectangular room.
From the Latin "pendens," which means "hanging."
In masonry the pendentives receive the weight of the dome, concentrating it at the four corners where it can be received by the piers beneath.
The dome rests on four pendentives. In pendentive construction, which apparently was developed after many yeas of experiment by builders in the Near East and constitutes the contribution of Byzantium to architectural engineering...
By transferring the weight to piers rather than to the wall itself, pendentive construction makes possible a lofty, unobstructed interior space...
The domes of earlier, central plan buildings, like the Pantheon and Santa Costanza, spring from the circular bases of a continuous wall or arcade.
The pendentive system is a dynamic solution to the problem of setting a round dome over a square or rectangular space, making possible a union of centralized and longitudinal or basilican structures...
- Gardner's Art Through the Ages, Tenth Edition, by Richard G. Tansey and Fred S. Kleiner. Harcourt Brace College Pub. 1996, p. 291
Examples from Buffalo architecture: