An EMF meter is a scientific instrument for measuring electromagnetic field

Electromagnetic field

An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...

s (abbreviated as EMF). Most meters measure the electromagnetic radiation

Electromagnetic radiation

Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 flux density

Flux density

-Formal Statement:The flux density is simply defined as the amount of flux passing through a unit-area. -Mathematical Statement:The flux density would essentially be the number of field lines passing through a defined unit-area...

 (DC fields) or the change in an electromagnetic field over time (AC fields), essentially the same as a radio antenna, but with quite different detection characteristics.
There are many different types of EMF meters, but the two largest categories are single axis and tri-axis.