1. To open by force.
2. To enter someone's property in an unauthorized manner, usually with the intent to steal or commit a violent act; SYN. break.
3. To intrude on uninvited.
4. To make submissive, obedient, or useful, as of wild animals or new items; used metaphorically for people; SYN. break.
5. To start in a certain activity, enterprise, or role; to familiarize one with such a role; .
6. To use something new (such as clothing, or a vehicle) for a period during which the object undergoes certain changes; .

1. The act or action of breaking in
2. A performance or a series of performances serving as a trial run
3. An initial period of operation during which working parts begin to function efficiently

When you break in a new mechanical device or a car, you use it slowly and carefully until you are sure it ready for heavier use. When you break in a pair of shoes, you wear them only occasionally and for a short time until they are comfortable. When you break in people at a new job, you train and supervise them and give them less than the normal amount of work until they are ready for something more difficult.