You will have more luck getting a roomful of Biologists agreeing on a definition for the word ‘Alive’, or Logicians for the word ‘Reason’, than you will with a roomful of Psychologists defining the word ‘Subject’.
The Bio-Engineer knows it is a silicon-chip atop the neurons and between the firing synapse. The Geneticist sneers at this simplification at what is clearly a Gene [imminent in its discovery]. [DNA, perhaps?]
We won’t even broach the Mystics for now. But a particularly famous one from India is: ‘The sense of ‘I-ness”, which means whatever you want it to mean. [Inner-Self? Spirit? Soul?]
We go low-tech. We ask the Grammarian.
‘The Subject of a sentence is the person, place, thing, or idea that is doing or being something. It is what acts or is acted upon.’
Ego is Latin for ‘I’. The Cambridge definition reads: ‘Your idea or opinion of yourself’. Note the self-loop.