Bottleneck.
That's something that slows down the flow of info. So if a cpu can put out 150fps and you have a 144Hz monitor yet at ultra the gpu can only render 60fps, the gpu would be considered a bottleneck.
Doesn't really apply to cpus. The cpu is the source of fps. If you have a cpu that can only render a game at 60fps, yet have a gpu well capable of 150fps and a 144Hz monitor, the cpu is not slowing down the flow, it's simply not the flow you want. Doesn't make it a bottleneck, just a monitor and gpu more capable than the cpu. There are more than a few who say that's a cpu bottleneck, but it's misleading.
The biggest issue with bottlenecks is their absolute rarity. They actually almost never truly happen. Reason being is software. You can have one game so cpu intensive that you get low fps output, but swap that to a 4k monitor and the cpu is plenty, the gpu totally overwhelmed. Or a game that's graphically challenging, switch to a 1080p monitor and the cpu now struggles. All it takes is the wrong game paired with one component to change everything.
I play Starwars online. Single player or 8man team, no issues, great fps, 90+. Same game, same settings, 24man boss fight and I can drop to 5-15fps. Did my cpu suddenly become a bottleneck with just a arena choice?