Please answer a question about the following article about Antenna (radio):

The radiation of many antennas shows a pattern of maxima or "lobes" at various angles, separated by "nulls", angles where the radiation falls to zero. This is because the radio waves emitted by different parts of the antenna typically interfere, causing maxima at angles where the radio waves arrive at distant points in phase, and zero radiation at other angles where the radio waves arrive out of phase. In a directional antenna designed to project radio waves in a particular direction, the lobe in that direction is designed larger than the others and is called the "main lobe". The other lobes usually represent unwanted radiation and are called "sidelobes". The axis through the main lobe is called the "principal axis" or "boresight axis".

What is the term that refers to areas where an antennas radiation is zero?
nulls
The article indicates that a lobe of maximum radiation occurs when the radio waves emitted by different parts of an antenna arrive at a distant point in phase, while nulls occur where they are out of phase. 
Therefore, based on this information we can determine that "nulls" refers to areas where an antennas radiation is zero..