REFERENCE

We have to define reference as an act by wich speaker (or writter) uses language to enable a listener (o reader) to identify something. 
To perform an act of reference, we can use:

• Proper nouns: As Chomsky, Jennifer, Whiskas...
• Other nouns in phrases: A writter, my friend, the cat...
• Pronouns: He, she, it...

We sometimes assume that these words identify something or someone uniquely, but is more accurate to say that, for each word or phrase, there is a "range of reference".