London Dry Gin is a style and quality designation of gin. A London Dry Gin must be column distilled from a base spirit of agricultural origin, and then redistilled in the presence of juniper berry and other botanicals, have specified (very low) levels of methanol, not be colored in any way, and dry, that is totally unsweetened.
The official definition of London Dry does not actually specify which botanicals must be present, but it does specify that the predominant taste must be juniper, and that flavors cannot be added after distillation.