Noun

1. The shell of a clam.
2. A dredging bucket with hinges like the shell of a clam.
3. In foodservice, the white, usually styrofoam box given to consumers with take-out food.
4. An object which has a hinge on one edge and two surfaces that close together

   It closes like a clamshell.

5. An amphitheater, especially an outdoor amphitheater; the semi-circular acoustic backdrop behind and above the performers.

Adjective

1. similar to the operation of a clam's shell, having a hinge with two sides closing together

   It's a clamshell antenna