expansion joint

from The Century Dictionary.

• noun In steam engineering: Any kind of joint for connecting steam-pipes which permits the pipe to expand or contract under varying temperatures without increase of its length over all.
• noun An attachment of a boiler in its framing to allow the former to expand without affecting the latter.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun construction A structural separation between two building elements that allows free movement between the elements without damage to the system.