A plectrum (plectra, or plectrums in plural - or, if you're in the US, a pick) is in a very general sense anything that can be used to pluck, strum or pick strings of musical instruments, such as the harpsichord, the balalaika, the shamisen, the bouzouki, the mandolin or the electric guitar. The handheld plectrum is - in contrast to the built-in plectrum of the harpsichord - the most visible, and common type of plectrum.
There is a huge variety of plectrum designs. The most common of them, the guitar plectrum, which is more commonly called a 'pick', or more specifically a 'flatpick', is triangular piece of plastic with rounded-off edges, between one and two centimetres long (sizes, shapes and materials may of course vary). Also very common are the banjo plectra, which are called 'fingerpicks'. This variation of plectrum is sometimes also made of metal. It is usually the same size as a common guitar plectrum, but two corners are lengthened and wrapped around forming a ring so it can be worn on the thumb (or on all fingers) of the banjo player. Electric bass plectra are usually longer, and more rarely used. Shamisen plectra are called 'bachi'. They are about three times bigger than guitar plectra and are made of horn or ivory. Until the 16th Century the lute was played with a quill plectrum, but around 1500 this was abandoned in favour of a finger style (based on harp technique) similar to that now used by classical guitarists.
The thickness and consistency of the plectrum affects the sound of the instrument in combination with the player's picking techniques. Thin and flexible plectra usually produce sharper tones and allow for a wider range of sound dynamics, going from soft to loud, whereas thicker plectra produce a fuller, rounder sound. Some guitar teachers recommend using heavy or thick plectra to practise picking; when going back to the normal type of plectrum the velocity and the precision is increased. Some say that dynamics (ie, the alternation between loud and soft notes or the accentuation of the notes) are best practised with soft or light picks; when going back to normal picks the dynamic becomes more pronounced. However, different players might have different experiences.
Assorted knowledge associated with plectra or picks
The word 'plectrum' comes from the Greek 'plektron' meaning 'something to strike with'. It was used to pluck or strike the strings of the lyre and the kithara. In antiquity plectra were made of tortoiseshell, ivory, wood, metal, etc. (in fact they can be made of any solid material). Medieval and Renaissance writers used the term 'plectrum' to denote the 'bow', such as the bow of a violin, as there was no Latin word for 'bow'.
Plectra share a property with remote-controls, bottle-openers, socks, lighters, keys and important documents, namely the tendency to disappear when needed most. For this reason the ancient Greeks kept the plectrum (inelegantly) attached to the instrument (lyre) with a string.
Sometimes picks fall into the soundhole of the acoustic guitar. Getting them out is not complicated: Hold the instrument with the soundhole facing the floor, shake the instrument thoroughly until the pick falls out of the soundhole.
The Arabic oud1 is played with a plectrum. Medieval Arabic writings describe plectra made from eagles' talons as well as ones of wood, ivory, bone, tortoiseshell and quill. Before the invention of plastic, tortoiseshell was also widely used for making guitar plectra. Harpsichords were voiced with quills, and for a softer sound leather plectra were also used.
One of the many rituals of a rock concert is that the guitarists will throw their picks into the crowd. Some bands have personalised, custom-made or signed picks which go on to become valuable collectable memorabilia (an online collection can be seen at Band Guitar Picks).
As mentioned before, virtually any solid material can be used to function as a plectrum. Brian May, for example, is notorious for using sixpence coins, and Billy Gibbons was seen using flattened quarters (25 American cents). In an extreme sense, Jimi Hendrix's teeth could also be seen as occasional plectra. Some Researchers reported the use of mikado sticks, credit cards and even a toy pizza that came along with a Barbie kitchen-set.