The Free On-line Dictionary of Computing (30 December 2018):
1. A processor design which is not intended to be
implemented as hardware, but which is the notional executor
of a particular intermediate language (abstract machine
language) used in a compiler or interpreter. An abstract
machine has an instruction set, a register set and a model
of memory. It may provide instructions which are closer to
the language being compiled than any physical computer or it
may be used to make the language implementation easier to
port to other platforms.
A virtual machine is an abstract machine for which an
Examples: ABC, Abstract Machine Notation, ALF, CAML,
F-code, FP/M, Hermes, LOWL,
Christmas, SDL, S-K reduction machine, SECD, Tbl,
Tcode, TL0, WAM.
2. A procedure for executing a set of instructions in
some formal language, possibly also taking in input data and
producing output. Such abstract machines are not intended to
be constructed as hardware but are used in thought
experiments about computability.
Examples: Finite State Machine, Turing Machine.