(Adjective) Used or devised to stop something from happening, or to stop people from doing a particular thing.
(Noun) Something that stops something unwanted from happening, especially something that protects against illness.
1) Done before others can act: Done before somebody else has had an opportunity to act so making his or her planned action pointless or impossible. 2) Intended to prevent attack: military intended to eliminate or lessen an enemy’s capacity to attack (a preemptive strike). 3) Discouraging further bidding
Here are a few examples.
In 1908 preventive detention was introduced to deal with `habitual criminals' who were dangerous to society.
The provincial government introduced a number of preventive measures, including extending the scope for blood-serum tests to identify people infected with HIV.
They were fully prepared to carry out 'pre-emptive action' to halt a potential Iraqi attack.
On 7 December 1941 the Japanese launched a pre-emptive strike on the Hawaiian base of Pearl Harbor, putting much of the US air and naval force there out of action.