uniformo

Here is a seam that is real in Latin and false in Esperanto, which is the trickiest kind. The word genuinely was built from unus plus forma, one shape, and the meaning follows perfectly: everybody wearing the same thing. But that building happened in Latin, centuries before Esperanto existed, and the word arrived here whole.

The proof is in the pieces. Esperanto has formo, so half the parse works, but the other half does not: uni- is not an Esperanto morpheme. The number one is unu, and a word built inside the language would have to say unuforma, which in fact exists and means uniform in the sense of unvarying, all of one kind. So you get two words from the same Latin idea: unuforma built here, uniformo imported ready-made. This is the inspiri class, where the international word carries a derivation that Esperanto could have performed itself and did not need to.

In practice uniformo is the garment: the soldier's, the nurse's, the schoolchild's. For the abstract sense use unuforma or unueca, and note that uniforma also circulates for uniform in the mathematical or statistical sense.