The launch window is a term used to describe a time period in which a particular mission must be launched.
An interval of time during which a space vehicle can be launched to accomplish a given mission, e.g. a flight to Venus or Mars.
Launch window is a term used in aerospace to describe a time period in which a particular rocket must be launched. If the rocket does not launch within the "window", it has to wait for the next one before it can be launched.