Up sampling vs down sampling,?
Within the context of Digital Signal Processing...
Upsampling and interpolation are synonyms, expander is not.
Specifically, an upsampler and an interpolator include an expander followed by an anti-imaging lowpass filter. The expander, is the initial stage of an interpolator or an upsampler.
The following is an expander:
$$x[n] \longrightarrow \boxed{ \uparrow 2} \longrightarrow y[n] = \begin{cases} { x[n/2]  ~~, ~~~n = 0,2,4,... \\ ~~~~ 0~~~~~~~ , ~~~n= 1,3,5,...} \end{cases} $$
The following is an upsampler / interpolator:
$$x[n] \longrightarrow \boxed{ \uparrow 2} \longrightarrow w[n] \longrightarrow \boxed{ h[n]}  \longrightarrow y[n] $$
Similarly, downsampler and decimator are synonyms, compressor is not.
The following is a compressor:
$$x[n] \longrightarrow \boxed{ \downarrow 2} \longrightarrow y[n] = x[2n]$$
and the following is a downsampler / decimator:
$$x[n] \longrightarrow \boxed{ h[n]}  \longrightarrow w[n] \longrightarrow \boxed{ \downarrow 2}  \longrightarrow y[n] $$
Specifically, a downsampler and a decimator include an anti-aliasing lowpass filter followed by a compressor. The compressor, is the final stage of a decimator or a downsampler.
Note that you may omit the anti-aliasing lowpass filter when downsampling (or decimating) due to the fact that the input signal is already bandlimited to the band of interest. In such a case, apparently the expander, the downsampler, and the decimator become the same operations; but this is not the general case.
There's no fixed definition of such things but the above is very common.
Authors might prefer using them in different contexts as long they do explicitly describe what they are doing. Informally I may use upsampler and expander in similar places, but the distinction should be clear in my mind.