A lineup is when people or objects are arranged in a row one after another. Think of a team standing in a line for a sports event.
When we talk about a lineup, we usually mean a group of people, like players on a team, waiting to perform or play. Imagine a baseball team taking the field - that's what you'd call a lineup. In a store, they might have a display with products arranged in a neat line, which could also be called a lineup. If police are showing suspects' pictures for identification, that would be a lineup too.
How common is "lineup"?
Word lineup is considered uncommon in modern English. It appears most frequently in newspaper texts and less often in other writings.
Definitions
noun
A physical or photographic queue of people allegedly involved in a crime, allowing a witness to identify them
A line of people or vehicles, in which the individual at the front end is dealt with first, the one behind is dealt with next, and so on, and in which newcomers join at the end; a queue.
Collectively, the members of a team.
Example: The manager fielded his strongest lineup for the game against United.
The batting order.
The members of a music group at any one time.
The acts performing at a concert or music festival.