LSL Wiki : collision

collision_start(integer num_detected)

This event is raised when another object or avatar starts colliding 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.

Example:

default {
    collision_start(integer num_detected) {
        llSay(0, llDetectedName(0) + " collided with me!");
    }
}

collision_start() and Linked Objects:
In a linked object, place a collision_start 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_start event in the child.
To pass collisions on to the parent from a child with a collision_start event, use llPassCollisions.

Compare with collision and collision_end.

Events | Collisions

LSL Wiki : collision_start