Technical vocabulary is words or phrases that are used primarily in a specific line of work or profession.
For example, people who work in the steel industry often use words like "Rockwell", "Olsen", "cup test", and "camber". These words have special meanings pertaining to the manufacture of steel.
Similarly, an electrician needs to know technical words such as capacitor, impedance, and surge capacity; words which most people outside of that industry never use.