Conjunction

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

   If it rains, I will get wet.

2. Supposing that; used with past subjunctive indicating that the condition is not fulfilled.

   I'd prefer it if you took your shoes off.

3. Although; used to introduce a concession.

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

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

   If A, then B, else C.

5. Whether; used to introducing a noun clause as the object of certain verbs.

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

Noun

1. An if clause representing a condition; a protasis introduced by if.
   • 1709, Susannah Centlivre, The Busy Body, Act III, in John Bell (ed.), British Theater, J. Bell (1791), page 59,

     Sir Fran. Nay, but Chargy, if——— ¶ Miran. Nay, Gardy, no Ifs.——Have I refus'd three northern lords, two British peers, and half a score knights, to have put in your Ifs?

   • 1791 January, "Richardſon’s Chemical Principles of the Metallic Arts", in The Monthly Review, R. Griffiths, page 176,

     Well might Bergman add, (in his Scicgraphia,), “if the compariſon that has been made, &c. be juſt.” The preſent writer makes no ifs about the matter, and has ſuperadded a little inaccuracy of his own, […]