Consciousness
The ability to be aware of internal or external existence. This is a really tough nut to boil into a paragraph and it’s notoriously ambiguous. It’s worth pointing out that “awareness of your awareness” is likely only a subtle variety called self-consciousness. Read more here
Sentience
The ability to experience feelings and sensations. Depending on how you qualify it this can be a very general category.
The broadest definition simply requires that sensation is registered and responded to (Like paramecium: poking induces mechanoreception induces thigmotaxis). The stricter definition requires a subjective valence to this perception (Like humans: being poked induces mechanoreception induces feeling pain induces running away)
Sapience
The ability to act rationally, make intelligent decisions, and non-obvious insights. This is not a binary category, but some helpful tests include “Can the individual recognize itself in a mirror”, “Can it disseminate and receive learnings from other individuals”, “Can the individual exercise good judgement between multiple options”.
Several particularly smart species can meet many or most of these criteria: dolphins, apes, elephants, corvids.
Salience
Neuroscientific attentional mechanism that identifies relevant stimulus and focuses their cognitive resources on it. It is the ability for something to “stand out” in an individual’s mind. A bright color on a dull landscape, loud noise in quiet area, quick movement on a still scene. It is the ability to identify, process, and react to these events.
Generally this will require a CNS.