Clamshell - definition
The phone consists of two halves connected with a hinge in the middle. The hinge allows the phone to be folded close (much like a sea clamshell, hence the name). When the phone is closed, the keyboard is protected from accidental key presses.
The top half usually hosts a small external display and a large internal main display, while the bottom half incorporates the keyboard.
The camera (if present) can be on either half.