While most monsters will appear promptly after a certain amount of time has passed, some monsters wait an additional, random, length of time before spawning. This range of time is sometimes referred to as a spawn window.
Timed Spawn monsters have finite spawn windows and may have more than one window.
Lottery Spawn monsters do not have finite spawn windows, but some require a certain amount of time to pass before they can be spawned again. This can be referred to as either the monster's "minimum respawn time" or as the monster's spawn window opening.