Single stage-to-orbit, also known as SSTO, was a type of space vehicle.
Unlike space vehicles which used booster rockets to get into space, an SSTO vehicle used engines such as the aerospike engine. This was an advantage for the SSTO as these vehicles used less fuel. These reusable vehicles could take off vertically and land horizontally.
An example of an SSTO was the space shuttle orbiter OV-165 from 21st century Earth. (PIC: "Two of One")