neŭtrala

This is the word the anthem is built on. The third verse of La Espero opens:

Sur neŭtrala lingva fundamento, komprenante unu la alian, La popoloj faros en konsento unu grandan rondon familian.

On the foundation of a neutral language, understanding one another, the peoples will form one great family circle. Zamenhof wrote it in 1890 and the movement has sung it at every world congress since, so of all the ranks in this band this is the one most Esperanto speakers could quote without thinking. Until today the word had no page and the song had to declare it as new vocabulary.

What neutrality was supposed to mean. Not that the language has no politics, but that it belongs to nobody's state. No army enforces it, no ministry of education owns it, no economy is served by teaching it. That claim carried real weight in a Europe where language choice was a live territorial question, and it is why the movement adopted the posture of a neutral country: the Universal Esperanto Association ran its wartime correspondence service through Geneva precisely because Switzerland was neutral and letters could cross the front there when they could cross nowhere else.

And the honest limit. The vocabulary is overwhelmingly Romance with a Germanic and Slavic seasoning, so Esperanto is markedly cheaper to learn for a Portuguese speaker than for a Japanese one. It is neutral between English and French. It is not neutral between English and Japanese. That criticism is addressed at more length in the introduction to this course, and pretending it away does the language no favours.

Grammatically the word is unremarkable: an ordinary -a adjective, neŭtrala, neŭtralaj, neŭtralan. Neŭtraleco is neutrality as a quality and neŭtraligi is to neutralise.