materio

Materio is the physical stuff of the universe, the thing physics weighs and chemistry breaks up. It is not the English matter that means an affair or a business: that is afero, and mixing the two is the single most common mistake English speakers make with this root. Pri kiu afero vi parolas? asks what the issue is. Pri kiu materio would be asking which physical substance you mean.

It is also worth keeping apart from materialo, which is a material in the builder's sense, the wood or brick or cloth you make something out of. Materio is the abstract category; materialo is what is in the yard. The two roots are the same Latin word, split in Esperanto so each does one job.

There is no -eri- hiding in this word. Learners who have met -er-, the particle suffix that turns sablo into sablero, a grain of sand, reasonably wonder whether materio is mat plus something. It is not. Materi- is one indivisible borrowed root, and the -i- belongs to it rather than to any affix. This is the false decomposition trap, and it catches the learner who has understood the affix system well enough to start taking words apart on sight.