In agency law, an individual or other legal person represented by an agent.
In law, a legal person acting on their own behalf.
The amount of an investment or a loan, excluding any interest. When the whole of a loan is drawn down at the start, the principal is simply the amount originally borrowed.
The reference amount of a traded financial instrument, used to determine its future cashflows.
Most important, or largest.
The concepts of 'principal' above, should not be confused with principle, which is different.