From Longman Dictionary of Contemporary Englishprotractorpro‧trac‧tor /prəˈtræktə $ proʊˈtræktər/ noun [countable] 
HMa piece of plastic in the shape of a half-circle, which is used for measuring and drawing angles → set square
HMa piece of plastic in the shape of a half-circle, which is used for measuring and drawing angles → set squareExamples from the Corpus
protractor• Moira screamed and ran away from him, dropping several books and a protractor.• After all, this is the unit used on the protractor that we first came across early in our school careers. 