An eccentric muscle contraction is one is which a muscle produces force whilst it is actively lengthening.
Example of eccentric muscle contraction
A good example is when your quadriceps muscles (front of thigh) eccentrically contract and play out to control your leg when you walk downstairs.
Eccentric Muscle Contractions: Risks and Benefits. Hody S, Croisier J-L, Bury T, Rogister B and Leprince P. Front Physiol. 2019; 10: 536.
Combination of Eccentric Exercise and Neuromuscular Electrical Stimulation to Improve Biomechanical Limb Symmetry After Anterior Cruciate Ligament Reconstruction. Lepley LK, Wojtys EM and Palmieri-Smith RM. Clin Biomech (Bristol, Avon). 2015 Aug; 30(7): 738–747.