Question about NF ieee and NF dsb

Hi, there
From spectre document, we can find the definition for Nsi & Nso:
Nsi = noise at the output due to the image harmonic at the source
Nso = noise at the output due to harmonics other than input at the source
but really I dont know what is the image harmonic at the source, what does the "image harmonic" mean? Also, which component at the source is not the input but the harmonics as the Nso definition says.

Thanks.

Generally speaking:
"harmonics" are coherent and integer multiple of the wanted and observed signal.
But often, in the microwave slang, someone call "harmonic" every CW signal present on the spectrum.

Your specific case:
regarding image (Nsi):
taking in example a common down converter mixer,
It convert the "wanted" RF freq (freqLO+freqIF) , but also it will convert the "image" RF Freq (freqLO-freqIF). The mixer convert both the RF freq. and the user will chooses what is image and/or wanted.

regarding "harmonics" (Nso):
a "perfect2 mixer give te following outputs:
fIF= fRF-fLO
fIF=fLO-fRF

but practical mixer on the market will produces output also for:
fIF=N*fRF-M*fLO
fIF=M*fLO-N*fRF
where N and M are positive, integer, >0 .
Of course as N and M rises, the power level decreases.