EIN is a specification that is mostly relevant to microphones and preamplifiers. It stands for Equivalent Input Noise and deals with the self noise that is a feature of virtually every piece of electronics. Since microphones output a low-level signal, any noise present will be boosted proportionately. The specification is usually measured with a 200ohm source resistance, which on its own produces a level of noise that when referenced to a line level signal is equal to -129.6dBu of noise. This is theoretically the lowest noise level that can be obtained, but some manufactures play with this figure a little by measuring their equipment with lower source impedance.