In climatology, the term megathermal (or less commonly, macrothermal) is sometimes used as a synonym for "tropical."
In order for a particular place to qualify as having a megathermal climate, every single month out of the year must have an average temperature of 18°C or above.
Megathermal climates are sometimes split into two temperature-based subsets — equatorial and tropical (the latter used here in the sense of "outer tropical") — with "equatorial" denoting little or no variation in temperature throughout the year and "tropical" denoting significant seasonal variation, even though no month has an average temperature of below 18°C. In addition, what temperature fluctuations do exist in an equatorial climate will typically bear no relationship to the astronomical seasons for the applicable side of the equator, while in the (outer) tropical subtype the temperature will move in concert with the seasons, the time of higher sun and longer days being warmest and the time of lower sun and shorter days coolest.
These climates can also be subdivided on the basis of rainfall, as examples of humid, semiarid and arid places can all be found within the megathermal category (although the region of arid megathermal climate is small compared with the extent of deserts existing in other climate zones, particularly the neighboring subtropical zone).