1. Something produced by chemical alteration of a natural starting material. The most common examples are antibiotics such as nafcillin, a semisynthetic penicillin derivative, and cefotaxime, a semisynthetic cephalosporin. Small changes in the functional groups of a naturally occurring substance can result in a new compound with enhanced properties. Nafcillin does not occur naturally, but is also too complex to be economically synthesized from scratch. Nafcillin is therefore produced by taking a naturally occurring penicillin produced by large-scale fermentation processes and making minor changes to the chemical structure.
2. A mixture of synthetic and natural components. Commonly seen on bottles of motor oil which contain both refined oil from natural sources and stuff the petroleum company cooked up in the lab. Many other types of lubricants as well as some coolants are also marketed as semisynthetic blends.