the.com/expansion joint

means A built-in gap or flexible seam in a structure—bridge, sidewalk, pipeline, building—that lets materials expand and contract with temperature without cracking or buckling.

from Straightforward engineering English, not folklore: 'expansion' from Latin 'expandere' ('to spread out'), paired with 'joint' from Old French 'joint' ('joined, connected'), rooted in Latin 'jungere' ('to join'). The term rose with 19th-century railroads and bridge-building, once engineers realized that steel rails and iron spans grew measurably longer in summer heat and needed somewhere to push the slack — so they built in gaps on purpose, turning a flaw into a feature.