致函 is a formal verb meaning 'to send a letter'—typically in official, business, or diplomatic contexts. The character 致 (zhì) means 'to deliver,' 'to present,' or 'to direct toward,' while 函 (hán) means 'letter' or 'written communication,' historically referring to letters enclosed in a case or envelope. Together, they form a compact, literary expression emphasizing purposeful, respectful correspondence—not casual emails or texts.

This term appears frequently in official notices, corporate communications, government documents, and academic settings. It’s almost always used transitively, followed by the recipient (e.g., 致函贵公司 'send a letter to your company') or with an object like 一封公函 ('an official letter'). While modern usage sometimes includes email, 致函 retains its formal register and would never be used for personal messages among friends or family.