A concentric muscle contraction is one in which the muscle shortens as it contracts.
Concentric versus eccentric
If you lifted a heavy pan up from the cooker the biceps would contract concentrically; if you slowly lowered it down again the same muscle would contract eccentrically.
Comparison of eccentric and concentric isokinetic exercise testing after anterior cruciate ligament reconstruction. Yoon T and Hwang J. Yonsei Med J. 2000 Oct;41(5):584-92.