High: an area of the atmosphere where the pressure is high relative to that in the surrounding region at the same level. It is always associated with anti-cyclonic circulation (clockwise in the Northern Hemisphere); same as an anticyclone.
Low: a region of the atmosphere in which the pressures are lower than those of the surrounding regions at the same level. Since a low is always associated with cyclonic circulation (counterclockwise in the Northern Hemisphere) the term is used interchangeably with cyclone. It is also called depression.