WordNet sense 1 (capital as contrasted with the income derived from it):

corpus, principal sum

WordNet sense 2 (an actor who plays a principal role):

lead, star, lead, lead

WordNet sense 3 (the educator who has executive authority for a school):

head, head teacher, school principal

WordNet sense 4 (the major party to a financial transaction at a stock exchange; buys and sells for his own account):

dealer

WordNet sense 5 (most important element):

chief, main, primary

WordNet sense 6 (of great significance or value):

of import