Sprite Status Table (SST) is a portion of RAM which is allocated to a single object and stores information about that object. In Sonic games, the information stored includes the object's ID or starting code pointer, a pointer to its mappings, and other miscellaneous variables such as the current mapping frame, status bitfield, X and Y positions, etc.
SST is actually a misnomer, since objects and sprites are two different things in Sonic games (for example, an object may be represented by multiple sprites). The term was coined by Yuji Naka in patent number 5411272, filed in 1993. "Object Status Table" and "Object RAM" are sometimes preferred by the community. The SST should not be confused with the Sprite Attribute Table, which is a list of visible sprites in the VRAM.
In Sonic the Hedgehog and Sonic the Hedgehog 2, a single object is allocated $40 bytes of object RAM, and the format of this RAM is very similar between the two games. Sonic the Hedgehog 3, however, allocates $4A bytes to a single object, and the format differs greatly from the previous two games. Sonic & Knuckles, and consequently Sonic 3 & Knuckles, both use Sonic 3's format.