The phrase "concentric part" refers to a component or a piece that is located inside or within another object, with both sharing the same center point. Full definition
The tempo for the lifts should be 2 -1-1, meaning two counts down (eccentric part of the lift), a one count pause, and one count up (concentric part of lift). (build-muscle-and-burn-fat.com)
and do both the eccentric and concentric parts of the exercise with as perfect a form as you can muster, and you won't get your skull crushed, per set. (fitnessandpower.com)
Always make sure you execute the exercises with a proper form with a tempo consisting of 3 seconds for the concentric part of the exercise and 4 seconds for the eccentric part, putting a bigger emphasis on the latter. (fitnessandpower.com)