noise figure (NF)
A single number that says how much a circuit dirties the signal passing through it. Every amplifier, no matter how good, adds some of its own random hiss; noise figure measures exactly how much the signal-to-noise ratio gets worse from input to output. A perfect, noiseless block would have a noise figure of 0 dB; real RF blocks add a fraction of a dB up to several dB.
Formally, the noise factor F is the input SNR divided by the output SNR, and noise figure is just that ratio in decibels (NF = 10·log₁₀F). It is referenced to a standard temperature of 290 K so everyone compares apples to apples. In a receiver chain, the front stages' noise figures cascade by Friis' formula, which is why a 1 dB difference in the LNA's NF can mean the difference between hearing a distant transmitter and hearing only static.
Don't confuse noise figure (a dB ratio) with noise temperature (kelvins) — they describe the same thing two ways; satellite engineers prefer temperature, IC designers prefer NF.