Lateral separation is a term that is more used in the air-traffic control area of aviation then in actual flight. Though it can be used by pilots to describe distances from points or objects it is mainly used by air-traffic controllers to maintain separation between aircraft flying on parallel courses. In order for the safety of flight to never be in question the air traffic control system must maintain separation between flying in the air at all times. One of the ways order is brought to the national airspace system is by the establishment of airways. These are designated highways in the sky in which a magnetic course is flown using the help of a NAVAID or satellite to guide a plane from one destination to another. Since these airways are obviously used by more then one airplane separation is required for the safety of flight.
There are many factors that determine how much lateral separation is needed. Since aircraft performing approaches and departures in and out of airports are the most critical times of any flight it is the most critical time when separation is need. When performing simultaneous approaches into an airport aircraft obviously they cannot land at the same time so they use 15-degrees of separation based on flight paths from converging aircraft. Your speed also can determine how far across you are from another aircraft. For example an aircraft traveling up to but not including the speed of sound will be separated by 1-degree of latitude.
With pilot lateral separation will most likely be used when referring to the distance from a fix, NAVAID, or obstruction. Towers, buildings, and geographical formations can all pose a threat to aviation and so the Federal Aviation Administration defined the minimum distance an aircraft must keep from those objects. Lateral separation can also be defined by airspace. When a pilot is flying an aircraft in or around particular airports airspace they must keep a certain distance from different areas of the airspace. This distance is published in aeronautical charts, publications, and NOTAMs (notices to airmen). Lastly lateral separation can refer to the distance a pilot must keep away from weather phenomena such as clouds or thunderstorms. Specific lateral separation distances are defined by the Federal Aviation Administration in the Federal Aviation Regulations handbook.