Rosin refers to an extraction process that utilizes a combination of heat and pressure to nearly instantaneously squeeze resinous sap from your initial starting material. When compared to BHO (butane hash oil), the two are aesthetically indistinguishable. Rosin, when made properly, retains just as many valuable terpenes that account for aroma and flavor. However, in a lab test, rosin will never contain a single parts per million (PPM) of residual hydrocarbon. In other words, you are essentially getting shatter without any solvents when using this process.