(baseball) a list of batters in the order in which they will bat
"the managers presented their cards to the umpire at home plate"
a line of persons arranged by police for inspection or identification
"The detective asked each suspect to step forward into the lineup so the victim could identify who took her purse."
a physical or photographic queue of people allegedly involved in a crime, allowing a witness to identify them
"The detective showed the witness a lineup of five suspects so she could point out her attacker."
In plain English: A lineup is when people are arranged side by side so others can pick out who they saw doing something wrong.
"The sports announcer described the starting lineup before the game began."
Usage: Use lineup when referring specifically to the police procedure where suspects stand together so an eyewitness can make an identification. Do not use this term for any other type of arrangement or sequence of events.
Lineup is a noun formed directly from the verb phrase to line up. It entered English as a convenient way to refer to the act of arranging people or things in an orderly row.