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

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; .

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.