Definition
The maximum slip angle a pilot can hold in a given airplane before running out of rudder authority, before control becomes uncomfortable or unsafe, or before exceeding the manufacturer's published slip limitations. Beyond this point, further increases in slip angle are not achievable or not advisable, even though more sideslip might theoretically increase drag and descent rate.
Plain English
The point past which you can't safely or physically slip the airplane any harder. Either the rudder is already on the stop, the airplane is getting hard to control, or the manufacturer says don't go further.
Context Anchor
You meet this term when learning intentional slips, especially forward slips used to increase descent without gaining airspeed.
Derivation
"Practical" here means real-world, usable in flight — as opposed to theoretical. The practical slip limit is what the airplane and the pilot can actually achieve, not what physics alone would allow.
Why Pilots Care
It defines the safe boundary for using a slip to lose altitude quickly or correct for wind without risking a stall or loss of directional control.
Intuition Check
Do not read “practical” as “rough” or “unimportant,” and do not read “limit” as necessarily meaning a damage point. Here it means the real usable point where more control input no longer gives a better controlled slip.
Example Sentence 1
On final approach the pilot held a forward slip right up to the practical slip limit to lose the extra altitude before the threshold.
Example Sentence 2
In a strong crosswind, the pilot held the slip just inside the practical slip limit to keep the airplane aligned with the runway centerline.