A "lunch break" is some time in the middle of the day when a worker can stop working and eat lunch. For a lot of jobs, the time and length of the lunch break is specific:

Our lunch break starts at 12:30.

I only get a half hour lunch break, and sometimes even that gets cut short!

If someone works in a job with a very flexible schedule, they probably don't talk about their lunch time as a "lunch break". Instead, they might say that they "take lunch" or "have lunch":

I usually take lunch at twelve thirty or noon.

In other words, the phrase "lunch break" sounds like a break which is controlled by the company or by a manager.