High key
I realise the use of it may vary slightly from type to type but generally it is a term used when doing a PFL pattern. It is roughly equivalent to the downwind call (low key or final key may be used for the finals call). The orientation may differ between aircraft but it is designed to allow a constant glide angle to be maintained from overhead the runway until touchdown.
I’m sure others can provide more detail for specific aircraft types. I can only speak for the Hawk. Our High Key (for a circling PFL) is 90 degrees off at the upwind threshold.
BV