A one way is something that travels from one location to another, without necessarily returning back again. A common abbreviation for one way in the travel industry is O/W. The standard opposite of a one way is round trip.
One way often refers to transportation, where travel is booked from Point A to Point B only, often called a one-way ticket.
One way is usually the preferred spelling for the noun, while the hyphenated “one-way” is often preferred as a modifier. However, along with the rarely-used, single-word “oneway,” these spelling variations are often used interchangeably.