Invalid argument

An argument that is not valid is said to be invalid.

By the definition of logical consequence, $A_1,...,A_n/\models B$ if there exists an interpretation $v$ such that:

$$v(A_i)=1\text{ for all }1\le i\le n,\text{ but }v(B)=0$$

Hence, in order to show a conclusion does not follow from a set of premises, we must find an interpretation that makes all of the premises true, but under which the conclusion is false.