Windshear is defined as a sudden change of wind direction or speed. From a pilots point of view you could add "that causes an unexpected change in airspeed or vertical speed". Any more than this is delving into more than just a definition.
Not too hot on the actual definitions etc, but here's a general description.Windshear most usually manifests as a change of wind velocity as altitude changes. The wind speed is usually stronger at altitude than close to the surface. The direction usually backs about 20 degrees at altitude compared with the surface, also, but this is extremely variable and depends on local effects (terrain) and the weather pattern/air stability on the day. The general descriptor of this is wind gradient, where the windpeed increases steadily and somewhat predictably with altitude. Shear is the more sudden relative. Normally windshear is most significant on approach, where the aircraft is close to minimum speed, and as it descends enters an area of less headwind. Because the aircraft has inertia, this translates to a decrease in airspeed. If severe enough the aircraft may not be able to recover from the situation (Talking pretty darned severe, here, but it has caused accidents.) The usual procedure where windshear on approach is reported or expected, is to carry a bit more airspeed. The usual procedure when severe windshear is encountered is to raise the nose and apply power, lots of power, as required. Naturally the nose shouldn't be raised so much the aircraft risks stalling, it's both intuitive and counter intuitive to do this. Intuitive (for the less experienced) because the aircraft is sinking below the normal profile; counterintuitive because it's unnatural to raise the nose if the airspeed is dropping. I believe it's been recommended as a windshear procedure, because it slows the descent rate initially, which is important, as the windshear is quite likely to get worse the faster altitude changes. It affects all aircraft. Because light aircraft have less inertia, usually a lower descent rate on approach, and are less often landing on runways that are close to the minimum length than large airliners, it's usually not so critical. But definitely can be. All that said, windshear can occur at any altitude, in any flight condition, given the right (wrong) met conditions. Often associated with jetstreams. Almost always occurs near/in thunderstorms.
Windshear. (Shearing action-two air masses, or parts of one air mass, shearing against each other.) Not to be confused with Windsheer. (Slightly provocative, see-through air.) or Windshare (the malodorous gift that no one really wants to share.)
I'd add to backpacker's reply. Beware coastal airfields where the prevailing wind is off-shore, but the sea-breeze is producing an on-shore wind at ground level. Often gives a shear layer at low altitude which is more of a concern on departure than approach, as the headwind turns to a tailwind with consequent loss of IAS. On approach you may suddenly become hot due to the reverse effect. Also generally more noticeable at night due to reduced vertical motion and mixing (a 30-40knot wind at 1000' might turn to 5-10 knots at GL), though the shear is likely to be more progressive and easily handled.
shear vs. sheer. I guess it all depends on whether you are from the UK or the US, whether you're a native English speaker or English is your second language, whether it's late at night or not, whether your typing skills are up to scratch or impaired by alcohol intake etc.
In any case, I do apologize to US English speakers, and apologise to English English speakers for my mistake.
Although my dictionary gives a second definition for "sheer" as well - to swerve from a course. So arguably a wind shift could also be called a windsheer instead of a windshear.
Windshear is, at it's simplest, two layers of air moving at different speeds, hence with a "shear" or tearing layer between them. It can also be a local up or downdraft, and associated in/outflow. Regardless, it's a pseudo-stable condition - if there was a downdraft of strength X 30 seconds ago, it's probably still there, and of similar magnitude.
In practice, one rarely sees windshear without some level of turbulence; the academic windshears used for training and system design therefore comprise both a pure shear model and a turbulence model.
AC120-41 contains some discussion of the modelling approach used.
The worst windshear I have experienced was on approach to GQNN, a couple of years ago. The First officer had made rather a mess of the first hand flown ILS approach, and was 'done in' so he said. Lockheed to the rescue. Couple it all up for an automatic approach/land and with calm wind reported at the surface, the FMS showed a crosswind of fifty knots at 400 agl. Drift angle, 12 degrees. The 'ole TriStar comes sliding down the glidepath at Ref+40, and at 5 feet radio height, the speed was right at Vref.
...or those 3 lovely engines were at idle on touch down...?
Actually, nicely spooled-up. The 'ole Roller is a fine engine, especially when the IAS drops like a rock, and you need the fast acceleration. The Lockheed autothrottle/autopilot systems work well in adverse situations...just as intended.