A Jansky is the unit of radiation density, or flux, usually associated with radio emission. It was named in honour of Karl Jansky, the first radio astronomer.

The Jansky or Jy has the units 10^-26 Watts Metre^-2 Hertz^-1 or

10^-26 W m^-2 Hz^-1

Although this seems very small, radio sources of milliJanksy or mJy are routinely detected by modern radio telescopes.

Some common units are:

nJy = 10^-9 Jy
μJy = 10^-6 Jy
mJy = 10^-3 Jy
kJy = 10³ Jy