Jitter
Also said as Packet delay variation, Unstable ping
How much the delay changes from one packet to the next. Packets sent on a steady beat arrive unevenly, and some overtake each other on the way.
A steady 100 ms can be planned for. A connection that swings between 40 and 160 is much harder, and it often plays worse, because whatever is built on the timing keeps being wrong.
Say it
“Add a jitter buffer that sits at 2 updates and grows only if more than 1% of packets arrive late. I would rather be 30 ms further behind than have other players stutter.”
Where you’ve seen it
Riot's Valorant write-up says buffering smooths out uneven packets but "effectively serves as extra network latency", so its targets are about half a frame of buffer on the server and one on the client.
Not to be confused with latency
Latency is how long the trip takes. Jitter is how much that time changes. A connection can have low ping and bad jitter, and that is what wifi often gives you.