Ownership interest in a corporation in the form of common stock or preferred stock. It also refers to total assets minus total liabilities, in which case it is also referred to as shareholder's equity or net worth or book value.
In the context of a futures trading account, it is the value of the securities in the account, assuming that the account is liquidated at the going price. In the context of a brokerage account, it is the net value of the account, i.e. the value of securities in the account less any margin requirements.
In real estate, it is the difference between what a property is worth and what the owner owes against that property (i.e. the difference between the house value and the remaining mortgage or loan payments on a house).