demisii

Intransitive, and the direction matters. You demisias yourself out of a job. You cannot demisii somebody else: sacking a person is eksigi, and the two are not to be swapped, because the entire political interest of the word lies in who did the deciding. The noun is demisio, the resignation itself, whether that means the letter or the act. Prezenti sian demision is to tender it.

A borrowed root doing a job the affix system could already do. The Reta Vortaro says as much and gently prefers the built alternatives: eksiĝi, from eks- meaning former plus -iĝ- meaning to become, so literally to become an ex-holder of the post; or eksoficiĝi, spelling out the office as well. Both are transparent to anyone who knows the affixes, which is the argument for them. Demisii is transparent to anyone who knows French, which is the argument against. In practice the international press style of Esperanto journalism uses demisii constantly, so you will read it whichever side you take.

Not the same as rezigni. Rezigni is to give up or renounce anything at all, and it takes pri: rezigni pri la vojaĝo, to give up on the trip. Demisii is narrower and formal, and it applies only to a post or an office. A minister who rezignis might merely have abandoned an argument. A minister who demisiis has gone.