Exercise where the muscle changes length against a constant external load, which is what almost all free-weight training is.
Every set of curls with a 20 kg barbell is isotonic: the load stays 20 kg, the muscle shortens on the way up and lengthens on the way down. It sits alongside isometric (length fixed) and isokinetic (speed fixed) as the three classic contraction types.
The catch is that constant load does not mean constant effort. The biceps is far weaker with the elbow near full extension than at 90 degrees, so a 20 kg curl is near-maximal at the bottom and easy at the top. That is why the hardest part of a rep is always one specific joint angle.
Bands and chains exist to fix this. Adding a band to a bench press makes the load rise as the lifter gets stronger through the range, so the effort is closer to even from chest to lockout.