If

noun

An uncertainty, possibility, condition, doubt etc.

1. Supposing that, assuming that, in the circumstances that; used to introduce a condition or choice.

   If it rains, I shall get wet.

2. In the event that a statement is true (a programming statement that acts in a similar manner).

   If A, then B, else C.

3. Supposing that; used with past or past perfect subjunctive indicating that the condition is closed.

   I would be unhappy if you had not talked with me yesterday.

4. Supposing that; given that; supposing it is the case that.

   If that's true, we had better get moving!

5. Although; used to introduce a concession.

   He was a great friend, if a little stingy at the bar.

6. (sometimes proscribed) Whether; used to introduce a noun clause, an indirect question, that functions as the direct object of certain verbs.

   I don't know if I want to go or not.

7. (usually hyperbolic) Even if; even in the circumstances that.
8. Introducing a relevance conditional.

   I have leftover cake if you want some.