For our purposes, a trade most closely fits Merriam-Webster's third definition: "an occupation requiring manual or mechanical skill." These jobs are sometimes called "skilled trades" in recognition of the specialized knowledge they require; each occupation requires an extended period of training to practice safely and effectively. This broad definition means that a wide range of jobs are considered trades. They each have different education and training requirements, and some call for certification or licensure, but we can give a few general principles about what's required.