n. 25B6; adjective the principal cause of poor air quality : MAIN, chief, primary, leading, foremost, first, most important, predominant, dominant, (most) prominent; key, crucial, vital, essential, basic, prime, central, focal; premier, paramount, major, overriding, cardinal, pre-eminent, uppermost, highest, top, topmost; informal number-one.
the principal of the firm : CHIEF, chief executive (officer), CEO, chairman, chairwoman, managing director, MD, president, director, manager, head; informal boss; Brit. informal gaffer, governor.
the school's principal : HEAD (TEACHER), headmaster, headmistress; dean, rector, chancellor, vice-chancellor, president, provost; N. Amer. informal prexy.
a principal in a soap opera : LEADING ACTOR/ACTRESS, leading player/performer, leading role, lead, star.
repayment of the principal : CAPITAL (SUM), debt, loan.