1. To use an electronic tracking system to fix onto a particular person or thing. Once the satellite locks in on the rocket's signal, we should be able to triangulate its precise location. With proton beam therapy, we lock in on the cancerous cells and deliver radiation only to the affected tissue.

2. To fix the electronic tracking system onto a particular person or thing. In this usage, a noun or pronoun is used between "lock" and "in on." The fighter pilot locked his missiles in on the enemy jet. We should be able to lock our homing system in on Jeff's tracking unit.

3. To focus one's gaze or attention completely on someone or something. She locked in on the handsome guy across the bar, making her way over to talk to him. Once she locks in on a new project, there's no getting in his way.