Electromagnetism. Any frequency in the range of 0 Hz - 100 kHz.
The classification of non-ionizing electromagnetic fields into low frequency and radio frequency is made according to scientifically documented interaction with the organism. Below a threshold of 1 MHz stimulation or excitation effects of nerves, muscles, and sensory receptors may occur. Above 1 MHz only thermal effects have been scientifically proven yet.
In other context, low frequency refers to the frequency range 0 - 100 kHz.