What is NXT's "lessor"? While reading NXT's API wiki page I came across something called a "lessor", but I can't find its definition anywhere. What is a lessor?
A lessor, in general, is a person who leases a property to another.

NXT allows you to "lease" your forging ability to someone else.

So getAccountLessors gets a list of people who have leased their forging power to your account.