an instrument pick that's (you guessed it) flat.

A triangular or teardrop-shaped piece of nylon or plastic used to pluck or strum guitar strings. Flatpicks are available in a large variety of shapes, sizes, and thickness.

Flatpick: a small, flat object, usually plastic, held between the thumb and forefinger or index finger, and used to strike the strings of an instrument; also known as a pick or plectrum.