Purpose of the word in this context

What is the purpose of the word “il” in this sentence:

Lei `e il mio collega.

It's a definite article, and you'll see il mio, la mia, etc (per the gender; singular or plural) used a lot in Italian.  I hope this helps clarify https://grammar.collinsdictionary.com/italian-easy-learning/possessive-adjectives  Per the article, possessive adjectives agree with the noun they go with, not with the person who is the owner.

Chris

Hi Nicole, 

Thanks for your question :) There isn't much I can add to the clear and comprehensive explanation provided by Chris, except that Italian articles identify the gender and number of nouns (and they can be masculine or feminine, singular or plural). 

Since “collega” is a noun (colleague), it must be preceded by an article. In this case, the article is “il” (masculine, singular) because in this case specifically the pronoun “Lei” is used as “formal you” referring to a male colleague. The phrase can be translated as “You are my colleague.”

Let's take another example: 

“Lui è il mio insegnante” - “He is my teacher” ----→ as you can see, the noun “insegnante” is preceded by the article “il” because the noun is masculine and singular.

“Lei è la mia professoressa” - “She is my professor” ---→ as you can see, the noun “professoressa” is preceded by the article “la” because the noun is feminine and singular.

Hope this clarifies your doubts, but feel free to share any additional questions and I'll be happy to help! :)