Canonical refers to something that follows the law stated by the canon, canon refers to the bible in general. In this context, non-canonical translation means a deviation from the general known rules of translation.

viruses are under continuous selection pressure to keep their genome sizes to a minimum, viruses have evolved such novel translation strategies in order to thrive in the host cells.

@Vamsi Bharadwaj SV : Thanks very much for the detailed and lucid explanation sir.

It also conveys non-mainstream. You may have heard "canonical signalling", which basically means signalling pathways that are deemed by experts to exist.

It means the main subject, the main title or the main address in religious Christian beliefe.