An elimination is a reaction in which an organic compound loses two ligands without gaining any.

eg. 1:

net reaction:

In this reaction, 1 loses two ligands, H and Br, without gaining any. Therefore, this reaction is an elimination reaction.

eg. 2:

net reaction:

In this reaction, 2 loses two ligands, H and Cl, without gaining any.  Therefore, this reaction is an elimination reaction.

Overall, elimination is the exact opposite of addition.

eg:

see also 1,1-elimination, 1,2-elimination