Page "Nyquist frequency" Paragraph 5

In theory, a Nyquist frequency just larger than the signal bandwidth is sufficient to allow perfect reconstruction of the signal from the samples: see Sampling theorem: Critical frequency.

However, this reconstruction requires an ideal filter that passes some frequencies unchanged while suppressing all others completely ( commonly called a brick-wall filter ).

In practice, perfect reconstruction is unattainable.

Some amount of aliasing is unavoidable.