五角形（ごかくけい、gokakukei）
- pentagon

The Japanese word 五角形 (読み方：ごかくけい, *gokakkei*) means "pentagon" or "pentagonal shape." It refers to a polygon (a two-dimensional shape) that has five sides and five angles. It's a fundamental geometric shape. A 五角形 can be regular (all sides and angles are equal) or irregular (sides and angles are not all equal).

⚫︎その建物は五角形をしています。 Translation: That building is pentagonal. ⚫︎五角形のテーブルは部屋の隅にぴったり収まりました。 Translation: The pentagonal table fit perfectly in the corner of the room.

You use 五角形 when you're talking about anything with a five-sided shape. This could be in geometry class, when describing the shape of something, or even in art. For example, if you're talking about a pentagon-shaped sign or a pentagonal building, you'd use this word. The word functions as a noun.

⚫︎The word is typically written with the kanji 五 (go, meaning "five"), 角 (kaku, meaning "corner" or "angle"), and 形 (kei, meaning "shape"). ⚫︎The reading uses hiragana: ごかくけい. ⚫︎There is no variation in okurigana (the suffix hiragana).

There aren't many direct synonyms in common Japanese usage, as the concept is very specific. You might describe the shape using more descriptive language depending on the context if you're trying to avoid the specific word 五角形.

There isn't a direct antonym for 五角形. You would use words describing shapes with a different number of sides like: ⚫︎三角形 (sankakkei) - triangle ⚫︎四角形 (shikakkei) - quadrilateral

この図形は五角形］ですか？ ・Translation: Is this figure a pentagon? ・Explanation: This sentence directly asks if the shape in question is a 五角形.

学校で五角形の面積を計算する方法を学びました。 ・Translation: I learned how to calculate the area of a pentagon in school. ・Explanation: This sentence states that the speaker learned about 五角形 and calculations in school.

The word 五角形 is a compound word. 五 means "five," 角 means "angle" or "corner," and 形 means "shape." Therefore, the word literally means "five-cornered shape," accurately describing its geometric properties. The word is derived from Chinese, like many other mathematical terms in Japanese.