The phrase "concentric portion" refers to a part of something that is arranged in a circle or rings, with each ring being smaller and inside the previous one. Full definition
Sandhu et al. (2008) found that the triceps muscle activity was only greater during the eccentric rather than concentric portion of the press up with the hands on a swiss ball compared to on a bench. (strengthandconditioningresearch.com)
Forced reps: once you reach muscle failure during a set, have a spotter assist you in completing the concentric portion of a few more reps. Forced reps will work great for bench presses, bicep curls, squats and triceps extensions, but are best avoided on deadlifts, barbell rows and dips. (fitnessandpower.com)
Make sure to always perform the negative, eccentric part of the lift in a very slow and controlled manner, and allow for an equally slow and smooth transition to the positive, or concentric portion, but then lift the weight up explosively to recruit as many muscle fibers as possible. (fitnessandpower.com)