Quantum electrodynamics in the 1920s

Quantum electrodynamics is often shortened to QED. As for what it is exactly, I find it difficult to say. Wikipedia says it’s the relativistic quantum field theory of electrodynamics, and gives “a complete account of matter and light interaction”. But that’s not enough. The Encyclopaedia Britannica says it’s a quantum field theory which “describes mathematically not only all interactions of light with matter but also those of charged particles with one another”. That’s not enough either. Particularly since it’s defining QED in terms of other things…