关于sampling

有没有达人，介绍一下以下一些东东：
undersampling: the sampling frequency is lower than the nyquist rate,but higher than the maxium signal frequency
over sampling: the sampling frequency is much higher thant the nyquist rate
nyquist sampling : the sampling frequency isbased on the sampling theory( two times the maximum signal frequency)
subsampling : the sampling frequency is lower thant the maximum signal frequency
baseband sampling :the signal to be sampled is basebandsignal (i.e. signal with spectra centered around DC)
bandpass sampling: the signal to be sampled is passband signal
are these right?

I believe they are correct. However, when use sub- or under-sampling, the sampling frequency still has to be at least twice of the signal bandwidth to meet the Nyquist requirement.