kariero

The -er- you can see is a real suffix, and it is not in this word. Esperanto's -er- means one particle of a mass: sablo sand gives sablero a grain of sand, mono money gives monero a coin, fajro gives fajrero a spark. It is productive and you will use it. So kariero offers a parse: kari- plus -er- plus -o, one fragment of some kari substance. There is no root kari-. Karier- is a single borrowed unit, and the -er in the middle is French, arrived here by accident of spelling.

The etymology is a small delight. Latin carrus was a cart, which gave carraria, a road wide enough for carts, which gave French carrière, a racecourse. A horse ran a carrière at full speed, and by the seventeenth century a person's life could be described the same way: a course you run. English took the metaphor and buried it so thoroughly that career meaning "to hurtle" now sounds like a different word. The cart is also behind cargo, carry and charge, all of them things you load onto it.

Register and use. Kariero is a life's professional course rather than a job: profesio is the trade you are qualified in, posteno the post you currently hold, okupo your occupation, and laboro the work itself. Karierulo is a careerist, and in Esperanto as in English it is faintly rude. Fari karieron is to make a career, karieristo the same idea with the ambition showing. And ŝanĝi karieron is what you do when the first course was the wrong one.