A Radiation Pattern is a 3 dimensional description of how an antenna radiates power in the surrounding space. This pattern is usually measured at a sufficient distance from the antenna known as the far-field. In simple words it is the power radiated in a certain direction with reference to an omni-directional antenna (a theoretical antenna that radiates equally in all the directions). Given below are the radiation patterns for some common antenna types.
Although the Radiation Pattern is a 3 dimensional quantity it is usually sufficient to describe it in two orthogonal planes (one horizontal and one vertical) as shown in the figures above.
 Cisco Aironet Antennas and Accessories: Antenna Patterns and Their Meaning