field rate

The field rate of an interlaced video image is twice the effective frame rate, since interlacing draws only half of the image (the odd- or even-numbered lines) at a time. For example, a field rate of 60 Hertz will correspond to a 30 frames-per-second moving picture. A field rate can apply to either a video signal, or to a monitor designed to display that signal without conversion.
See more at Wikipedia.org...