Extension means 'straightening'.

Full extension is generally quoted as 0 degrees but may normally be as much as -6 degrees.
Range of motion
When the knee is straight it is said to be extended. A normal range of motion at the knee includes flexion, extension and a small amount of hyperextension.
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CLINICAL PUBLICATIONS
How to Rapidly Abolish Knee Extension Deficit After Injury or Surgery: A Practice-Changing Video Pearl From the Scientific Anterior Cruciate Ligament Network International (SANTI) Study Group. Delaloye J-R, Murar J, Sánchez MG, Saithna A, Ouanezar H, Thaunat M, Vieira TD and Sonnery-Cottet B. Arthrosc Tech. 2018 Jun; 7(6): e601–e605.