To gain strength and hypertrophy more load must be put on the bar, then we have volume, gradually building up more reps and sets... I guess load on the bar and volume are what most weight lifters have in thought when discussing how to progress for hypertrophy and strength. But what about "density" or doing the same amount of work in shorter time span? Doing 30 set in 45 minutes will represent a progression if you used to use one hour with same amount of weight. Progressing on "form" should also be counted... Curling 100 lbs in good or better form represent a progression in targeting your biceps if you are advancing from a rather sloppy cheating curl.
So it seems like we have at least four possible ways of doing "progressive overload": 1) Intensity or load on the bar, 2) Volume = more reps and set, 3) Density or the time factor and 4) Form or the technical aspect of the lifts = the mind-muscle connection.