Deliberate throwing overboard of cargo or parts of the ship If done in time of peril, this is a general average sacrifice but is also recoverable as a direct claim on the policy
Releasing an airborne weapon or store by means of an emergency or secondary release system for safety purposes or to prepare for air combat Weapons are normally jettisoned in a safe condition
If you jettison something, for example an idea or a plan, you deliberately reject it or decide not to use it. The Government seems to have jettisoned the plan. = abandon
To jettison something that is not needed or wanted means to throw it away or get rid of it. The crew jettisoned excess fuel and made an emergency landing. = discard