What is the definition of the word the?
Definition: a definite article, which hints precisely a certain object, a specific thing or even a determined person. It´s used for meaning that we are not speaking of ANY, but of SOME UNIQUE being - even if it´s also used for plural (not ANY, but SOME people/things/objects).
Examples (notice the difference of meaning):
1. Give me a CD please ( = any = no matter which or which type of CD);
2. Give me the CD please ( = that/this CD, not any other, only THAT/THIS one).