The part of a parachute deployment between when a canopy
leaves its bag and when it is fully inflated.
The length of a snivel is most effected by how well the slider keeps the packed canopy out of the wind. The slider is attached to the lines and is packed over the top of the open ends of the canopy. A larger and less porous slider lets less air directly into the ends of the canopy, increasing the snivel time. As the canopy inflates the lines are forced apart, pushing the slider down, which then exposes more of the canopy to wind.
Reserve canopies are designed to have a consistently short snivel. While some people like long snivels for soft opening main canopy, if a snivel is so long that the main has not inflated by the jumpers decision altitude they should consider it a malfunction, cutaway, and use the reserve. If the snivel is too short the opening can be dangerously fast with the potential for damaging a person or gear with more force then they are designed to withstand. Such slamming opens have never been fatal, but can leave you quite sore. Be careful when packing the slider, even if the rest of the pack job is untidy.
It is unknown why even the best jumpers with normally consistent snivels on the same gear, the same pack job, and same deployment position get sometimes get uncomfortably fast openings every 100 - 1000 jumps. As careful as you can be some things still come down to luck or lack there of.