w.e.f. - with effect from. Also w.i.e., with immediate effect.
WAAS - Wide Area Augmentation System (in USA)
wake turbulence - wingtip vortices generated behind a wing
producing lift. Behind a large heavy aircraft they can be powerful
enough to roll or even break up a smaller aircraft.
WAT - weight-and-temperature
wind shear - localised change in wind speed and/or direction
over a short distance, resulting in a tearing or shearing effect,
usually at low altitude, that can cause a sudden loss of airspeed with
occasionally disastrous results if encountered when taking off or
WIP - work in progress.
WP - waypoint.
wt - weight
Wx - weather.
WX NIL - no significant weather, term used in Met reports.