Formalise is the British spelling of formalize. To formalise something is to make it official or properly set — to take a loose idea, a handshake plan, or a habit, and turn it into rules, papers, or a clear form. A school might formalise a new timetable; two companies might formalise an agreement by signing it. It is a somewhat grown-up verb, more at home in offices, schools, and careful writing than in chat. Though actually, it does not mean “make fancy” or “dress in formal clothes”. Sometimes it just means “write it down so everyone has to follow it”. Americans write formalize; both spellings are the same word.

使正式化、把（口頭或鬆散的安排）定下來（英式拼法；美式作 formalize）