The gain of an antenna, especially on VHF and above, is quoted in dBi. The "i" in this expression stands for:

Antenna gain must be measured against a reference. In the term "dBi", the "i" stands for "isotropic", which refers to a theoretical isotropic radiator. An isotropic radiator is a theoretical point source that radiates RF energy perfectly equally in all possible directions, creating a mathematical sphere of radiation. Because this is a theoretical concept, it provides a standard mathematical baseline for comparing the directional gain of real-world antennas. Other options like "interpolated" or "ionospheric" are made-up terms in this specific context and do not represent antenna reference standards.