Flashes Per Minute, or FPM, is simply the number of times a flashing light blinks per minute

Minute

A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...

. Police and emergency vehicles use warning lights with flashing rates that typically fall in a range from 60 to 240 FPM.

Persons with photosensitive epilepsy

Photosensitive epilepsy

Photosensitive epilepsy is a form of epilepsy in which seizures are triggered by visual stimuli that form patterns in time or space, such as flashing lights, bold, regular patterns, or regular moving patterns.-Symptoms:...

 may suffer a seizure in response to a specific flashing frequency, e.g. 420 FPM, but the exact triggering frequency varies across individuals. To minimize the likelihood of an eleptogenic response, flashing rates above 300 FPM (5 Hz) should be avoided (Harding and Jeavons, 1994).

For conversion purposes, 60 FPM = 1 Hz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

(i.e. a frequency of one per second).