(Single Stage To Orbit) is a space launch vehicle that can achieve orbit without dropping off any hardware along the way; only propellent. It has never been achieved although it is very probably possible.
The reason it has never been achieved is that a rocket to reach orbit in a single stage requires more than ~92% of the vehicles mass to be fuel, due to the rocket equation, gravity losses and aerodynamic drag. This makes the tank and engines very difficult to build whilst handling and providing the accelerations needed to reach a stable orbit.
For example an ordinary drinks can is actually quite similar to some rocket fuel tanks, albeit much smaller, and is about 5% metal. But drinks cans lack rocket engines; adding a suitably sized engine would bring the dry mass to nearer 7-8%- leaving little or no space for any payload. Tanks made using wound carbon fiber rather than aluminum can greatly improve the 'dry mass' percentages however, and this allows for some payload again.
Several rocket designs exist that might well be able to reach orbit in a single stage, Roton, Black Horse, HOTOL being two very noteable examples.
It is frequently assumed that SSTO vehicles are necessarily reusable (i.e. it is a RLV), as the main point of making a vehicle SSTO is to make it simpler and easier to reuse. This is not strictly true however as leaving off the recovery systems can improve payload size and reduce vehicle costs (less moving parts). However, SSTO recovery systems can be smaller and lighter, because the rocket is so very light when empty. This lightness reduces the terminal velocity of the rocket has before touch down to maybe one hundred miles per hour, and reduces the weight of any heat shield; so the recovery system as a whole doesn't weigh so much. Still, SSTOs are going to be necessarily light and somewhat flimsy, and so this may impact how many times it may be reused.