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Reading wikipedia to be sure and it says:

In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled.

Is it any signals ? or just high frequencies aliasing as low frequencies ?

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You are basically right.

It's generally stated as the high frequency portion of the original signal spectrum that moves (as a result of the spectral shift caused by time domain sampling) into a new position which is interpreted as the location for low frequency signal spectrum regions. Henceforth a high frequency signal owning the identity of a low frequency signal. However, it's also possible for the low frequency portions of the original signal spectrum to shift into high frequency spectrum regions which would happen for example at a sampling rate of $F_s = F_c$ samples per second, where $F_c$ is the cutoff frequency in Hz, of the original bandlimited continuous-domain signal. Therefore it's just an aliasing; where any frequency component can move into any where in the spectrum according to the given set of parameters $F_s$ and $F_c$

Considering the interpretation of any signals; in a practical setting (a communications example) it can be observed where multiple independent communication channels are grouped together and may be resampled for some reaons which could result in aliasing between independent channels, hence any signals. But I wouldn't bother for this latter restricted definition of the term any signals.

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You a have a cable that represent the frequency axis of the spectrum of your signal, probably bounded (so, finite length cable). Then start to coil it up with some radius: frequencies at the same point in the circumference get added together in the digital spectrum, so you lose the individual information about each frequency.

The radius you choose is given by the sampling rate and determines which frequencies get added together. Ideally, you choose a radius such that only one turn is necessary so no frequencies overlap. If not, you get aliasing.

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