formation rule
rule for describing which strings of symbols formed from the alphabet of a formal language are syntactically valid within the language
en-US/fɔɹˈmeɪʃən ˈɹul/
en-GB/fɔːˈmeɪʃn ˈɹuːl/
rule for describing which strings of symbols formed from the alphabet of a formal language are syntactically valid within the language