The word "consciousness" is used in a variety of different ways, and there are large disagreements about the reality and nature (and even coherence) of some of the things people profess to mean by "consciousness."

Colloquially, the word "conscious" is used to pick out a few different things:

Wakefulness - The property that distinguishes, e.g., a person who is awake from a person who is asleep.
- We call people "unconscious" in this sense based on observed features like "sharply reduced mobility," though we wouldn't normally call someone unconscious if we think they're merely paralyzed. Instead, calling someone "unconscious" tends to imply reduced ability to perceive and/or reason about events in one's environment.
- An unconscious person (in this sense) might or might not be dreaming; and if dreaming, they might or might not be lucid.
Having experiences - The property that distinguishes, e.g., a comatose person who is having experiences from a comatose person who is not having experiences.
Knowledge, perception, and/or attention - E.g., we might say that someone becomes "conscious of" a fact when they first learn that fact. Or we might say that they become "conscious of" something whenever they're currently perceiving it, or whenever they're paying attention to it.
Meta-cognition or reflective awareness - Knowing, perceiving, and/or attending to your own mental states; or knowing, perceiving, and/or attending to the fact that you have certain mental states.
- E.g., we might say that someone is less "conscious" when they're fully immersed in a novel than when they're thinking about their own experiences, directing attention to the fact that they're reading a book, etc.
Self-awareness - Knowing, perceiving, and/or attending to your own existence or your own central properties.
- Depending on what exactly is meant by "self-awareness," the "immersed in a novel" example might also involve less self-awareness. In some weaker senses of "self-aware," one might instead claim that humans who are experiencing anything are always "self-aware."

Reasonably mainstream academic overviews of "consciousness" can be found in the Stanford Encyclopedia of Philosophy and the MIT Encyclopedia of the Cognitive Sciences.

This tag is tentatively and provisionally about the "having experiences" meaning(s) of "consciousness." For wakefulness and dreaming, see sleep. For knowledge, perception, and attentio...