Best definition
The wincentile was invented by Microsoft in the early 90s, but is now found in just about every progeny of

computer software. It is a percentile point which is bigger than the other 99, specifically one that takes a long time when a computer program is displaying its progress. Why these percentiles take longer than the other percentiles has always been a big mystery to end-users.
Usually a wincentile is the last percentile, just to cause the end-user additional stress and anguish, however wincentiles have been reported to occur earlier than that, especially in the Windows 95 installation process, where it is reported that several wincentiles even coexist in the same batch of 100.

Steven thought he would make it online in time to email his work in by the deadline, but that last wincentile took forever.