Wiktionary
n. The period between the earliest and latest times during which a rocket or spacecraft may be launched in order to achieve its mission
Wikipedia
In the context of spaceflight, a launch window is a time period during which a particular vehicle ( rocket, Space Shuttle, etc.) must be launched in order to reach its intended target. If the rocket is not launched within the " window", it has to wait for the next window.
For trips into largely arbitrary Earth orbits, any time will do. But if the spacecraft intends to rendezvous with a space station (such as the International Space Station) or another vehicle already in an orbit, the launch must be carefully timed to occur around the times that the target vehicle's orbital plane intersects the launch site.
Earth observation satellites are often launched into sun-synchronous orbits which are near-polar. For these orbits, the launch window occurs at the time of day when the launch site location is aligned with the plane of the required orbit. To launch at another time would require an orbital plane change manoeuvre which would require a large amount of propellant.
For launches above low Earth orbit (LEO), the actual launch time can be somewhat flexible if a parking orbit is used, because the inclination and time the spacecraft initially spends in the parking orbit can be varied. See the launch window used by the Mars Global Surveyor spacecraft to the planet Mars at 1.
To go to another planet using the simple low-energy Hohmann transfer orbit, if eccentricity of orbits is not a factor, launch windows are periodic according to the synodic period; for example, in the case of Mars, the period is 2.135 years, (780 days). In more complex cases, including the use of gravitational slingshots, launch windows are irregular. Sometimes, rare opportunities arise, such as when Voyager 2 took advantage of a 175-year planetary alignment (launch window) to visit Jupiter, Saturn, Uranus, and Neptune. When such an opportunity is missed, another target may be selected. For example, the ESA's Rosetta mission was originally intended for comet 46P/Wirtanen, but a launcher problem delayed it and a new target had to be selected (comet 67P/Churyumov-Gerasimenko).
Launch windows are often calculated from porkchop plots, which show the delta-v needed to achieve the mission plotted against the launch time.
Usage examples of "launch window".
Then we must work backward to a specific two-week period, and within each twenty-four hours we will have a launch window of exactly-there’.
Captain Aquino had used the intervening weeks to conduct as many tests and drills as were possible in the limited time available to him before the very tight launch window closed.
Because of a very tight launch window - less than an hour - they would recycle the countdown for a launch the next day between two and three P.
Lincoln, that your feeble attempt at sabotage cost us merely half an hour, not nearly enough time to cause us to miss our launch window.
They made it safely to the cell-ship, and then on out to the debris field and open space-all of it a blur, a necessary blur, a rush to make their launch window and return safely to Eclipse.
We've got to complete this job--and by that, friends, I mean complete it: get Hartnett up to optimum performance so that he can actually live on Mars--no back to the workshops if something goes wrong--in time for the launch window next month.
Even given the launch window constraints and all of that technical crap?