principal (investment, investor)

자료: Investopedia, http://www.answers.com/topic/principal

5. The main party to a transaction, acting as either a buyer or seller for his/her own account and risk.

cf: Other meanings:

1. The amount borrowed or the amount still owed on a loan, separate from interest.
2. The original amount invested, separate from earnings.
3. The face value of a bond.
4. The owner of a private company.

Can also be referred to as "corpus."

Investopedia Says:

Be sure to take into account the context in which this term is used, as the exact meaning of the term has many variations.