It's hard to decide what we need to define and what we don't. In this case, I kind of opted for assuming that the readers know what BPF is, given that a fairly high percentage of our articles seem to be about BPF these days. Still, I'll try to include a link next time, sorry.

Meanwhile, the LWN kernel index (https://lwn.net/Kernel/Index/#Berkeley_Packet_Filter) will lead you to more information about BPF than you ever wanted.

The target audience for LWN doesn't need eBPF defined for them.

This other article seems to explain it https://lwn.net/Articles/740157/

Its a pretty technical post, I'm guessing its intended audience knows what the BPF is?