Microstutter
Small, irregular variation in how long each frame takes to reach the screen — the image hitches even when the average frame rate looks high.
A game can report 60 frames per second on average while individual frames arrive at wildly uneven intervals — 10 ms, then 25 ms, then 8 ms. The eye reads the unevenness as a judder the average number hides. Causes include CPU/GPU pipeline contention, momentary hitches (asset streaming, garbage collection) and — famously — multi-GPU SLI/CrossFire setups where alternate frames came from different cards.
Fixing it is the job of frame pacing: deliberately delaying early frames so delivery is even. Tools like CapFrameX and PresentMon measure frame-time percentiles (1% and 0.1% lows) precisely because average FPS says nothing about stutter.