(evolutionary theory, geology) A sharp decrease in the total number of species in a relatively short period of time; specifically, a loss of ∼75% of all species on the planet over a geologically interval of less than 3 million years, as a result of a mass extinction event.

(physics, chemistry) The total number of protons and neutrons in an atomic nucleus. Symbol: A

(physics) The increased mass of an object moving at relativistic speed relative to an observer.

(physics) The mass of a body when it is not moving relative to an observer.

(chemistry) The mass of an individual molecule of an element or compound.

(physics) The amount of fissile material needed to support a self-sustaining nuclear chain reaction. / (figuratively) The tipping point.

(military) A body of troops held in military reserve by a general for striking a blow on an exposed flank or wherever the enemy reveals a weak spot.

(Christianity) A religious card that states that a Catholic Mass has been said in honour of a dead person, or as a request for special favours such as improved health or success in a worldly endeavour.

(geology) the movement downslope of soil and rock in response to gravity

(physics) the difference between the mass of an atom and the sum of the masses of its individual components in the free state; equivalent to the binding energy