'Planet' is Greek for 'wanderer' — five bright objects drifting across an otherwise fixed sky.
A celestial body orbiting a star, large enough to be rounded by its own gravity and to have cleared its orbital neighbourhood.
From Old French planete, from Late Latin planēta, from Greek planētēs (wanderer, wandering star), short for astēr planētēs (wandering star), from planasthai (to wander, roam). Planasthai derives from the PIE root *pleh₂- (to spread out, flatten), extended to mean wander or go astray, which also produced Latin plānus (flat, level — hence English plain, plane, explain, piano), Latin platea (broad way — hence English plaza, place, piazza), Greek platys (broad, flat — hence English platypus, literally flat-foot, and plate), and Old English feld (field). The Greeks distinguished planētai (wandering stars — the visible planets Mercury, Venus, Mars, Jupiter, Saturn, plus the Sun and Moon in ancient reckoning) from the aplanes asteres (fixed stars, the constellations). This distinction was fundamental to ancient astronomy and cosmology. The word entered Old English directly from Latin as planēta and was reinforced by Old French in the 13th century. The Copernican revolution redefined planet from wandering star to body orbiting the Sun, and the 2006 IAU decision further narrowed it to exclude Pluto, showing how the same word can be repeatedly redefined while retaining its etymological core. Key roots: *pleh₂n- (Proto-Indo-European: "to wander, to go astray (Greek planasthai; distinct from *pleth₂- flat/spread)").
The word 'planet' literally means 'wanderer' — and the same PIE root *pleh₂- (flat, spread) also gave us 'plain,' 'plane,' and 'explain' (to make flat/clear). Planets were the things that wandered across the flat sky, and explanations are ideas spread out flat so you can see them.