Drought by Tyra van Mossevelde

What is the definition of drought?

The drought problem due to climate change is frequently in the news. But what exactly is drought? When I consult the site of KNMI (The Royal Netherlands Meteorological Institute) and WMO (World Meteorological Organization) I find the definitions for:
1 - meteorological drought
2 - hydrological drought and
3 - agricultural drought.
The site of the NOAA (National Oceanic and Atmospheric Administration) also provides a definition for socioeconomic drought. But why is ecological drought outside the standard set of these major institutions?
I don't know the answer to that, but in my info drawing I placed it at the top.

(pencil on paper, 100x70 cm).