Definite description

In logic, a definite description is a term of the form 'the ...'. It is used to describe something. This is more difficult than it first appears, because:
Unfortunately, language allows to construct sentences, where the thing or person talked about is not unique, or it doesn't exist
Definite descriptions are called 'empty', if they violate one of the conditions above: Bertrand Russell used the sentence "The present king of Freance is bald.". When Russel wrote this, in the early 20th century, France was a republic, and had no king.
Other famous mathematicians who wrote about it include Gottlob Frege, and Peter Strawson.