Is mass a measure of inertia?

Mass is usually introduced as a measure of the inertia of a body. But what do we mean by inertia anyway? We introduce a measure of inertia. It turns out that for high, relativistic velocities neither the rest mass nor the relativistic mass fulfills the requirements for a meaningfully defined measure of inertia. But how are we going to talk about inertia in the physics lesson? How can we use students' everyday language and still arrive at a clear conceptualization? We will try to give an answer to these questions.