A plectrum, also called a flat pick, is a thin, curved piece of material, usually plastic, used to pluck or strum the strings of a guitar or similar instrument. Plectrums come in a variety of sizes, shapes, and thicknesses. Thicker ones are normally used for soloing and thinner ones for strumming.
A plectrum is normally held between the thumb and first finger. However, some guitarists hold it between their thumb and both first and second fingers for more control and accuracy of picking.
Gripping a plectrum puts strain on the muscles of the hand especially during a long performance. This can become more of a problem with increasing age but with modern manufacturing technology and advanced design, plectrums are available that ease the strain of playing.