To prevent or beat to the punch, to forestall an expected occurrence by acting first.
To displace or take the place of (by having higher precedence, etc).
To appropriate first.
To secure (land, etc.) by the right of preemption (purchasing before others, e.g. land because one already occupies it).
To make a preemptive bid at bridge.
A preemptive bid.
To take action in advance of an expected event