The Greek word “Tartarus” is only found once in the whole Bible. Tartarus describes the place, where a specific group of fallen angels are held until the final judgment. The verb tartaroo, translated "cast down to hell" in 2 Pet. 2:4, signifies to consign to Tartarus, which is neither Sheol nor hades nor hell, but the place where those angels whose special sin is referred to in that passage are confined "to be reserved unto judgment;" the region is described as "pits of darkness," RV.

2 Peter 2:4 NKJV
For if God did not spare the angels who sinned, but cast [them] down to hell and delivered [them] into chains of darkness, to be reserved for judgment;

Jude 1:6 NKJV
And the angels who did not keep their proper domain, but left their own abode, He has reserved in everlasting chains under darkness for the judgment of the great day;

There are differing opinions on what and where this place is. It could be an entire separate place from Hades and the Abyss or it could be an extension of either. It is not clear in scripture about this. Because there is no release until the judgment day I tend to think it is separate from the Abyss because in the Abyss there is coming and going of demons. I also tend to think it is not Hades, as Hades is the place of dead humans and there is no indication of demons being there. I would not keep myself up at night over this however.