What does enneagon mean?
An enneagon is a polygon with nine straight sides and nine angles, identical in meaning to the more familiar term "nonagon." When all nine sides are equal in length and all nine angles equal, it is called a regular enneagon, and each of its interior angles measures 140 degrees. The word comes from the Greek elements ennea (nine) and gonia (angle), making it a fully Greek formation alongside related terms such as polygon and diagonal. In practice, enneagon appears mostly in mathematical, educational, and architectural contexts, where writers sometimes prefer it to nonagon for its classical consistency. While rare outside technical settings, it is useful precisely because of that precision: anyone encountering it can be confident it refers strictly to the nine-sided figure.
nounA plane figure with nine straight sides and nine angles; a nonagon.
- A polygon with nine sides and nine angles.
"The floor of the pavilion was laid out as a regular enneagon, its nine sides radiating from a central skylight."
"The students constructed an enneagon using a compass and straightedge, then measured each of its interior angles."
"Each interior angle of a regular enneagon measures 140 degrees."
Standard plural formed by adding -s; used when referring to multiple nine-sided figures.
"Tessellations featuring enneagons require other shapes to fill the gaps between them."
"Enneagon" and "nonagon" mean exactly the same thing — one borrows pure Greek, the other a Latin-Greek hybrid — and mathematicians have argued for centuries over which deserves the name.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of enneagon
The word enneagon derives from ancient Greek: ennea, meaning "nine," combined with gonia, meaning "angle" or "corner" — the same root seen in polygon, pentagon, and trigonometry. It entered English as part of the wave of learned geometrical vocabulary borrowed from Greek, largely during the early modern period when European mathematicians were standardizing terminology. Its synonym nonagon blends the Latin novem- (via nonus, "ninth") with the Greek gonia, which is why some purists favor enneagon as etymologically consistent. Related English words sharing the ennea- root include ennead, meaning a group of nine.
How enneagon is actually used
"Enneagon" is a technical term found chiefly in geometry and mathematics writing; "nonagon" is considerably more common in everyday use. The plural "enneagons" or the Latinate "enneagona" may appear in scholarly texts, but "enneagons" is standard.
Easily confused with enneagon
An enneagon is a nine-sided geometric figure, whereas an enneagram is a diagram of nine points used in personality typology and spiritual teaching.
An octagon has eight sides, while an enneagon has nine.