Technically meaning the Jewel of four souls, it was *Accidentally* shattered by Kagome when she shot an arrow at it, so her and InuYasha have to reunite the pieces of it.
If Naraku got the Shikon no Tama, Everyone'd be screwed. 'Cept his detatchments.
Naraku: 'Cept you two. You betrayed me, so you're screwed too.
Kaseyu: Y'know, technically, I didn't betray you, I just refused to kill Pilmo.
Naraku: And therefore, you didn't follow orders, so, Technically, you betrayed me.
The jewel that was shattered by Kagome
in the anime series Inuyasha
. Literally means "Jewel of Four Souls" (Japanese)
If Kikyou gets the Shikon no Tama we'll all be screwed.