biologio

Biologio is the science of living things, and it is your gateway to a pattern that gives you dozens of words for free.

The -ologio family. Esperanto took the international -logi- ending wholesale, so the names of the sciences arrive in a block and each one is guessable from its subject: geologio, psikologio, sociologio, arkeologio, teknologio, zoologio. The practitioner drops the -i-: a biologo is a biologist, and biologia is the adjective.

Here is the catch, and it is the one the affix-like roots article exists to make: -ologio is not an affix. It behaves like one, it recurs like one, and the official list of affixes does not contain it. What is actually happening is that these words were borrowed one at a time from the same international stock, and their shared tail is a family resemblance rather than a rule you may apply. Coin *katologio for the study of cats and nobody will stop you, but nobody will recognise it either: the sciences that have -ologio names have them because the name was borrowed already assembled.

The test is the same one this whole band keeps applying. A real affix combines with any root you like (-ec- will take any adjective at all). An affix-like root only appears in words that were imported already containing it.