I've come across the following definition of plectrum in the Oxford Dictionary of English, which can be found at https://en.oxforddictionaries.com/definition/plectrum:
A thin flat piece of plastic, tortoiseshell, or other slightly flexible material held by or worn on the fingers and used to pluck the strings of a musical instrument such as a guitar.
This isn't an isolated use of the definite article with parts of the human body in this context. Other examples include definitions of shoe,
A covering for the foot, typically made of leather, having a sturdy sole and not reaching above the ankle.
What's the reason behind the use of the definite article in worn on the fingers, covering for the foot, and so on?
From what I know, the definite article is used when the reader is expected to know and recognize its referent – but I don't know what or whose fingers the definition makes reference to; the fingers don't seem unique or well-known, and neither they nor their possessor (the person or very adroit animal) is mentioned or implied – something that would otherwise help infer their existence. The same goes for the foot, etc.