1The kinship term for a parent's brother — what English calls 'uncle'. When written in kanji, 伯父さん is reserved for one older than the speaker's parent and 叔父さん for one younger; this distinction matters especially in formal documents like wedding/funeral announcements or family registers. In everyday speech and writing, おじさん in kana is the norm. Used both for one's own uncle and (with honorific さん) for someone else's uncle.
夏休みに田舎のおじさんの家に泊まりに行った。
Over summer vacation I went to stay at my uncle's place in the countryside.
父の兄なので、正式には「伯父さん」と書きます。
Since he's my father's elder brother, the formal way to write it is 伯父さん.
結婚式で、母方の叔父さんがスピーチをしてくれた。
At the wedding, my uncle on my mother's side gave a speech.
おじさんからお年玉をもらった。
I got New Year's money from my uncle.
2A familiar / vocative term for an unrelated middle-aged man. Typically used by children calling out to a stranger ('hey mister'), or by adults referring to a roughly 35-to-50-something man ('that ojisan over there'); younger than おじいさん (grandfather), older than お兄さん (young man). Speakers also use it self-deprecatingly ('I'm already an おじさん now'). Calling someone おじさん to their face can feel mildly rude if they don't see themselves that way. The word also fuels internet culture — e.g., おじさん構文, the stereotypically dated texting style of older men.
あそこのおじさんに道を聞いてみよう。
Let's ask that man over there for directions.
「おじさん、ボールが落ちました！」と子どもが声をかけた。
'Mister, your ball dropped!' the child called out.
もう40だし、すっかりおじさんだよ。
I'm already forty — totally an old man now.
「おじさん構文」って、絵文字とカタカナが多すぎるあれのことでしょ？
By 'ojisan-bun' you mean that texting style with way too many emojis and katakana, right?
3The Japanese common name for a saltwater fish, Parupeneus multifasciatus (manybar goatfish). The fish has long barbels (sensory whiskers) under its chin that are said to resemble an old man's beard — hence the name 'ojisan'. Found mainly in reef areas of Okinawa, Hawaii, and the wider Indo-Pacific. Normally written in katakana as オジサン.
沖縄の市場ではオジサンという魚がよく売られている。
At markets in Okinawa they often sell a fish called 'ojisan' (manybar goatfish).
オジサンはあごの下のひげが特徴で、その名前もそこから来ている。
The ojisan fish is known for the whiskers under its chin — and that's where its name comes from too.