Milliampere-seconds, also more commonly known as mAs, is a measure of radiation produced (milliamperage) over a set amount of time (seconds) via an x-ray tube. It directly influences the radiographic density, when all other factors are constant.

An increase in tube current (mA) results in a higher production of electrons that are inside the x-ray tube which will, therefore, increase the quantity of x-radiation; more radiation will mean more photons reaching the detector and hence apparent structural density will decrease, yet the signal intensity will increase. The time factor (s) is a measure of the electrons production duration in the tube; meaning 's' prescribes how long mA will last.

For example:   

mA x t = mAs (strictly mA.s but the "." is always omitted)
600 mA x 0.1 s = 60 mAs

Increasing either the current or time will increase the quantity of radiation; therefore the amount of radiation in an examination is represented as mAs.

The reciprocity law 

The reciprocity law states that the reaction of a photographic emulsion to light will be equal to the products of the intensity of that light and the time of the exposure ¹. This law pertains to mAs in the sense that all combinations of mA x seconds that amount to an equal quantity will produce the same amount of density.

50 mA x 0.2 se = 10 mAs is equal to 300 mA x 0.03 s = 10 mAs

It is due to this law that radiographers will have to take into consideration all other factors (mA, focal spot, source image receptor distance (SID), kVp) to reduce time to avoid motion blur.