Elision

In the words le, la, je, me, te, se, ce, ne, de, que (and other conjunctions ending in que), si (before il, ils), the final vowel is dropped before a word beginning with a vowel or a silent h and is replaced by an apostrophe: l'enfant, l'homme, j'ai, c'est, qu'il, puisqu'ils, s'il. This is called elision.