Science

That is true, the whole field is difficult to define using language.

Usually I interpret "consciousness' to mean 'qualia', but even this word is strongly debated by philosophers, so that doesn't help us much.

When talking about what is conscious and what isn't, I am usually talking about the notions of consciousness that are discussed when we look at the 'hard problem of consciousness'.

The definition of 'ability to react to the surroundings' is one that can be tested using scientific methods. When one applies a definition that is directly correlated to well-defined neural dynamics (brain activity in a specific region) or behavior (self-recognition when looking at a mirror), then those are usually referred to as the 'easy problems' of consciousness, because we can test these experimentally.

The hard problem deals with the actual subjective experience. Whether a 'movie' is being played for something or someone to experience.