ojisan

In Japanese, ojisan おじさん means "uncle," but sometimes refers to an older man.

Not to be confused with ojiisan おじいさん, or ojīsan with a macron, which mean "grandfather" instead.

Kanji

The word ojisan can be written with kanji in multiple ways, depending on the meaning of the word:

ojisan
伯父さん
Uncle, who is your parent's older brother. In other words, the oniisan of your otousan or okaasan.
ojisan
叔父さん
Uncle, who is your parent's younger brother. In other words, the otouto of your otousan or okaasan.
ojisan
小父さん
Elder man.

Variants

With family words in o__san お〇〇さん pattern, the o~ お～ prefix can be removed, and the honorific suffix can be changed between ~san ～さん, ~chan ～ちゃん, and ~sama ～さま.

	o~ Default.	∅ Relaxed.
~sama Respectful. Distant. Formal.	ojisama おじさま伯父様叔父様小父様	jisama じさま
~san Default.	ojisan おじさん伯父さん叔父さん小父さん	jisan じさん
~chan Chummier. Diminutive.	ojichan おじちゃん伯父ちゃん叔父ちゃん小父ちゃん	jichan じちゃん
∅ Relaxed.	oji おじ伯父叔父小父	ji じ

The words ossan おっさん, occhan おっちゃん, are other variants of ojisan.

vs. 伯父

The difference between ojisan and oji 伯父 (叔父) is that oji doesn't have honorifics.

Since you don't use honorifics toward yourself, when talking to other people about your family, you use oji 伯父 to refer to your uncle, and ojisan 伯父さん to refer to their uncle.

However, when talking to your own family, ojisan refers to your own uncle.

Toward Older Men

The word ojisan 小父さん can also refer not to your "uncle" but to an elder man instead. There's no hard rule about how old he has to be. It's a mid-point between oniisan and ojiisan.

おじさん　いくら何でもひどいと思いますほんとに･･････かける言葉もないわ･･････ポンフルフル

Manga: Hinamatsuri ヒナまつり (Chapter 9, 勘当ロックンロールフィーバー)

Context: Nitta Yoshifumi 新田義史 says what he did in a bar.
ojisan, ikura nandemo {hidoi} to omoimasu
おじさん　いくら何でもひどいと思います
[Mister], [no matter the circumstances] [I] think {[what you said] is horrible}.
- The bartender, who is a child, refers to Nitta as ojisan, because he's at an age around of her father's age, or her father's brother's age, that is, he could be her uncle if they were related, he's uncle-aged, ojisan-aged.
- ikura nandemo
  いくら何でも
  No matter how much, no matter the circumstances, regardless of what happened, etc.
honto ni..... {kakeru} kotoba mo nai wa......
ほんとに･･････かける言葉もないわ･･････
Really...... [I] don't even have [what to say]......
- honto - same as hontou 本当, with long vowel shortened
- kotoba wo kakeru
  言葉をかける
  To pour words [onto someone]. (literally.)
  To give someone words. To talk with someone.
pon
ポン
*tap*
(onomatopoeia.)
furufuru
フルフル
*shaking [head disapprovingly sideways]*
(mimetic word.)

In anime, sometimes child characters can call young men in their 20's ojisan, giving them a bitter and unwarranted taste of the cruelty inherent of the passage of time.

One particular anime about a salaryman ojisan: