In the sense of a
military weapon, a
conventional weapon is one which does not employ any of the
NBC triad (or, as
Neal Stephenson puts it,
NBCI quad) for its
destructive effect. This typically leaves
high explosive,
low explosive or
kinetic energy - or a combination thereof - as the
destructive agent.
Just in case, the NBC triad is Nuclear-Biological-Chemical. Neal's version includes Informational - like computer virii.