From Longman Dictionary of Contemporary Englishprimary stressˌprimary ˈstress noun [countable, uncountable] technicalSL the strongest force that is put on a part of a long word when you say it, like the force given to ‘pri’ in ‘primary’. It is shown in this dictionary by the mark (').
Examples from the Corpus
primary stress• When stress is being discussed, the mark indicates primary stress and indicates secondary stress.• When is primary stress placed on the first word of the compound and when on the second?• Obviously, single-syllable words present no problems - if they are pronounced in isolation they are said with primary stress. 