There are actually two terms you need to worry about: chromatic and diatonic.
Diatonic means "within the key". So, the notes that are diatonic to C major are C D E F G A B. The notes that are diatonic to E♭ major are E♭ F G A♭ B♭ C D. You dig?
Chromatic means "not within the key". So, in the key of C major, any note that's not C D E F G A B is chromatic. Similarly, in the key of E♭ major, any note that's not E♭ F G A♭ B♭ C D is chromatic.
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There are two types of chromaticism: functional and non-functional. You'll also hear essential and non-essential.
Functional/essential chromaticism: the chromaticism is part of a chord, i.e. a chord tone. So, an A7 chord in the key of C major would be chromatic because C# (the 3rd of A7) is chromatic to C major.
Non-functional/non-essential chromaticism: the chromaticism is not part of a chord, i.e. a non-chord tone. That same C# can be used as a chromatic passing tone between C and D. In this case, you might have something like a IV V progression (let's keep doing this in C major), where C is a chord tone on the IV chord (F), then you hear C# as a non-chord tone, and the V chord hits (G), and D is a chord tone. You could also have a chromatic neighbor tone: say we're staying on that V, and you have a figure going D C# D. C# is a chromatic neighbor tone in that instance. Of course, this is also possible with other non-chord tones.