Cannabis extracts are created with or without solvents. Concentrates made without solvents are considered solvent-less. (This usually refers to hash, rosin, or anything that used CO2 with no butane or propane.)
Properly made solvent-based concentrates have all of the solvents removed before being sold. Because of this, some companies will still label their concentrates to be solvent-less even though solvents were still used in the extraction process. If any solvent was used in the extraction process (even if 100% of it was purged out), it should be considered solvent-free, rather than solvent-less.
Most common solvents used: Butane, Propane, Alcohol, CO2, Ethanol.