aliasing
Aliasing is the corruption that happens when you sample a signal too slowly: frequencies above half the sampling rate fold down and impersonate lower frequencies, so a tone you never sent appears in your data — a fake, an 'alias.' The classic everyday demo is the wagon-wheel effect: a fast-spinning wheel filmed at 24 frames per second can look frozen, or even rotate backwards, because the camera samples it too rarely to track the true motion.
Once aliasing has happened it is permanent — the high frequency and its low-frequency alias become mathematically indistinguishable, so no amount of later processing can untangle them. That's why prevention is the only cure: an analog anti-aliasing low-pass filter is placed BEFORE the sampler to remove everything above the Nyquist frequency (half the sample rate). In images the same effect produces moiré patterns and jagged 'jaggies' on diagonal edges; in audio it produces eerie metallic tones that weren't in the original.
Anti-aliasing in computer graphics fights the same enemy by smoothing jagged edges, but there it's done after rendering; in signal acquisition the filter MUST come before the sampler, because once the alias is baked into the samples it can never be removed.