On DifferenceEngine, someone defined a computer as something that can "run stored programs that had conditional logic and iteration."
A different way to word that might be that a computer can:
If you're the kind of person who is attracted by the abstract and theotretical, a computer is often defined as an machine which can solve any problem solvable by a UniversalTuringMachine. This definition arose out of AlanTuring and AlonzoChurch 's formal definition of an Algorithm, and what kind of problems are Computable as opposed to notComputable.