From SKYbrary Wiki
Mass may be defined as a measure of the resistance of an object to change in its velocity.
The term 'mass' is often used interchangeably with 'weight' although strictly speaking the two terms are different. Weight is the force experienced by a mass under the influence of gravitational acceleration.
Mass and weight are measured in the same units. The standard measure of mass, which is used by the aviation industry in most countries, is the kilogram (kg). However, in North America, the non-standard alternative unit, the pound, is widely used.