For pipe expansion joints, this is the term.

BELLOWS. The flexible element of an expansion joint consisting of one or more convolutions and the end tangents, if any.

CONVOLUTION. The smallest flexible unit of a bellows. The total movement capacity of a bellows is proportional to the number of convolutions.
http://www.pipingtech.com/products/expjtcat/expansion_joint-...

A typical type of expansion joint for pipe systems is a bellows ... made up of a series of one or more convolutions, with the shape of the convolution designed to withstand the internal pressures of the pipe, but flexible enough to accept the axial, lateral, and/or angular deflections.
http://en.wikipedia.org/wiki/Expansion_joint