A working definition, and an honest caveat
Start with the plainest version: you are conscious right now. There is something it is like to be you reading this sentence, to taste coffee, to feel a stubbed toe. That felt, first-person quality is what most people mean by consciousness. The philosopher Thomas Nagel captured it in 1974 by asking what it is like to be a bat, and the phrase 'what it is like' has anchored the field ever since.
The caveat is that there is no single, agreed scientific definition. Researchers use the word for several different things: being awake rather than in dreamless sleep, being aware of the world, having a rich inner life, and being aware that you are aware. The Stanford Encyclopedia of Philosophy calls consciousness both the most familiar and the most puzzling feature of the mind. So a good answer names the distinctions clearly rather than pretending the debate is settled.
Ned Block's two kinds: phenomenal and access
The most useful distinction comes from the philosopher Ned Block, who separated phenomenal consciousness from access consciousness. Phenomenal consciousness is the raw experience itself: the redness of red, the ache of the ache. Access consciousness is information that is globally available to the rest of your mind, ready to be used in reasoning, speech, and deliberate action. Usually the two travel together, but Block argues they can come apart.
This matters for the science. Much of what brain research measures is access: whether you can report a stimulus, act on it, or hold it in working memory. Report is a clean signal, but Block warns that phenomenal experience may 'overflow' what you can report at any instant. Whether that overflow is real or an illusion of memory is still argued, and it shapes how experiments on consciousness are designed and interpreted.
Levels, contents, and states
Modern consciousness science tends to split the topic into three dimensions. Level is how conscious you are, from deep anesthesia through drowsiness to full alertness. Content is what you are conscious of: a face, a melody, a decision. State is the overall mode you are in, such as ordinary waking, dreaming, or a psychedelic experience. Keeping these apart prevents a lot of confusion, because a person can be low in level yet still have vivid content, as in dreaming.
These dimensions are measurable and increasingly clinically useful, and they are where empirical progress is happening. What they do not yet explain is the deeper puzzle: why any of this physical processing is accompanied by felt experience at all. That question, the so-called hard problem, is worth its own page rather than a footnote here, so we point you there instead of re-litigating it.