Glossary
What is wind shear?
A sudden change in wind speed or direction over a short distance, either horizontally or vertically. Because it changes the airspeed the wing sees, it can produce a rapid loss or gain of lift, and it matters most close to the ground where there is no height to trade.
That is the whole entry for now. The longer version, with the mechanics and worked examples, is still to be written. See autorotation for how those read once they exist.