LSL Wiki : collision

collision(integer num_detected)

Collisions raise this event when an object or avatar collides with the object the script is attached to. The number of detected objects is passed to the script in num_detected. Information on those objects may be gathered via the llDetected* functions. Collisions are also generated if an avatar bumps into or walks on top of an object.

The collision event is not raised when using llVolumeDetect, only collision_start and collision_end are.

Example:

default {
    collision(integer num_detected) {
        llSay(0, llDetectedName(0) + " is currently colliding with me!");
    }
}

collision() and Linked Objects
In a linked object, place a collision event in the root prim to affect the whole object, not just the root prim.
To prevent collisions on a specific child prim from being relayed to the parent, place an empty collision event in the child.
To pass collisions on to the parent from a child with a collision event, use llPassCollisions.

Compare with the collision_start and collision_end events.

Events | Collisions