Webster's Revised Unabridged Dictionary · 1913

If

If , conjunction

[Old English if, gif, Anglo-Saxon gif; akin to OFries. ief, gef, ef, Old Saxon ef, of, Dutch of, or, whether, if, German ob whether, if, Old High German oba, ibu, Icelandic ef, Gothic iba, ibai, an interrogative particle; properly a case form of a noun meaning, doubt (compare Old High German iba doubt, condition, Icelandic if, ef, ifi, efi), and therefore orig. meaning, on condition that.]

1.
In case that; granting, allowing, or supposing that; -- introducing a condition or supposition.
Tisiphone, that oft hast heard my prayer, Assist, if Odipus deserve thy care. — Pope
If thou be the Son of God, command that these stones be made bread. — Matt. iv. 3
2.
Whether; -- in dependent questions.
Uncertain if by augury or chance. — Dryden
She doubts if two and two make four. — Prior
Collocations (1)
As if or But if , See under As, But.