Open 
1. Not blocked or obstructed.
2. Not complete or requiring continued attention.
3. An electrical circuit that has a break preventing current from flowing through. 

Open Circuit 
An electrical circuit that has a break preventing current from flowing through. 

Open Cornice 
A cornice with exposed, rather than enclosed, rafter ends. 

Open-end Block 
Concrete block, which has an end removed so that it can be placed around vertical steel reinforcement. 

Open-End Lease 
A lease contract providing for a final additional payment on the return of the property to the lessor, adjusted for any value change. 

Open-End Loan 
A loan where the borrower may add to the principal without renegotiating the terms of the loan. Additional sums borrowed under the terms of an open-end loan will have the same rate of interest and life of loan terms as the original loan. A home equity loan is often open ended. 

Open-End Mortgage (Deed of Trust) 
A mortgage or trust deed that can be increased by the mortgagee (borrower). The mortgagee may secure additional money from the mortgagor (lender) though an agreement which typically stipulates a maximum amount that can be borrowed. 

Open-end Wrench 
A wrench with a head designed to fit over a specific size nut or bolt. 

Open Grain 
Coarsely-grained wood that may require filler or sealer before staining. Oak is an open grain wood. 

Open Hearth Furnace 
A furnace with a shallow hearth used in steel processing. The steel is directly exposed to the heat source. 

Open House
Method of showing a home that is for sale. The house is left open for prospective buyers to see. The house may be advertised in the newspaper and/or outside the house. 

Open Housing 
Housing where purchase or lease is available to everyone regardless of race, ethnic origin, color or religion and affirmative action is actively pursued. 

Open Listing 
Property marketed by a number of brokers simultaneously. This type of agreement permits a real estate agent to sell the property while allowing the homeowner of other agents to attempt to make the sale. 

Open Listing Agreement
Listing agreement given to many brokers and/or agencies. The property owner pays a commission only to the broker who actually produces a buyer for the property. 

Open Mortgage
Mortgage that has matured or is overdue and, therefore, open to foreclosure at any time. Alternately, a mortgage that does not have a prepayment clause and permits the mortgagor to repay the mortgage at any time without paying a penalty. 

Open Occupancy 
Housing where purchase or lease is available to everyone regardless of race, ethnic origin, color or religion and affirmative action is actively pursued. 

Open Space
Land, which is left undeveloped for use as parks, walking paths, etc., by those living in a planned community. 

Open Space Ratio 
The ratio, in a development, of open space to developed land. 

Open String Stairs 
A staircase where at least one end of the treads is visible.

Open Time
The period that a product can remain open to the air without impairing its usability. Examples to products with limited open times include paint, adhesive, and joint compound. Some products can be salvaged after they have exposure has exceeded the open time while others are ruined. 

Open Valley 
A style of roofing where valley flashing is left uncovered by shingles. Water flows from the shingles to the valley and is directed off the roof. 

Open Web Joist 
A metal joist where the edges are reinforced by a lightweight structural steel in a zigzag pattern, creating a strong yet lightweight framing member.

Opening Escrow 
The deposit given by a buyer of property is delivered to the escrow agent, who retains it for the seller. 

Operating Expense Ratio 
Mathematical equation obtained by dividing operating expenses by gross income. 

Operating Expenses 
Amounts paid to maintain property, such as property taxes, utilities, hazard insurance, repairs, maintenance, legal, management fees, etc. 

Operating Lease 
Regular rental of property between the lessee and lessor for a fee. An operating lease does not satisfy the criteria for a capital lease. 

Operating Leverage 
Automatic increases in the net operating income or cash flow of income-producing real estate when income and expenses increase at the same rate; further enhanced when expenses are fixed. 

Operating Statements 
Financial reports on the cash flow of a property. 

Operator 
An individual running or controlling a piece of machinery. Often refers to the driver of piece of excavation equipment. 

Opinion of Title 
A certification, usually from an attorney, as to the validity of title to property being sold, after examining the abstract of title. The opinion of title is essential to obtaining title insurance, a mortgage or to the transfer of title.

Opportunity Cost
A term used in economics; when taking a particular action, the loss of the value of the next best action. 

Optical Level 
A telescope with a level that is mounted on a tripod to enable the user to sight another point at the same elevation. see transit. 

Optical Plummet 
A device suspended below a transit allowing the operator to site the transit over a specific spot. 

Optics 
The science covering the behavior and characteristics of light. 

Optimize 
To increase the efficiency or effectiveness of a process as much as possible.

Option 
1. A document giving the right, but not the obligation, to purchase a piece of property at a predefined price and terms within a set time period. 
2. The ability or right to choose a certain alternative.
3. Features offered in a new home that are not included at the base price - for example, upgraded cabinets. 

Option Listing 
Listing agreement, which also gives the listing broker the right to purchase the property. 

Option Listing Agreement 
A contract, given for a consideration, where an optionor gives an option to the optionee for the right, but not the obligation, to purchase property within a certain period of time, at a certain price. If the option is not exercised within the specified period of time, it will expire worthless. 

Option to Purchase 
Contract that gives one the right, without any obligation, to purchase a property, within a certain period of time and at a certain price, subject to conditions. 

Optional 
A right to select something or perform some act; not required. 

Optionee 
One who receives or purchases an option 

Optionor 
Person or business that gives or sells an option.