dictionary-style conceptual definition, but in physics we need to back up a conceptual definition with an operational definition, which is one that spells out the operations required in order to measure the quantity being defined.

oncoming poodle than in the path of a freight train, because my body will have a harder time convincing the freight train to stop.

completely different approach is to measure how hard it is to change an object's state of motion.

set is a collection of objects