What does joon mean in Persian?
If you spend any time around Iranians, you will hear joon within minutes. It is one small word doing a lot of warm work.
Joon جون literally means life or soul. Jan جان is the same word in its more formal, written form; joon is how it usually sounds in everyday Tehran speech. When Persians attach it to a name, it becomes a term of affection, roughly "dear".
After a name: Sara joon
The most common use is straight after someone's name: Sara joon, Ali jan. It softens and warms the name, a bit like saying "Sara dear", except it is completely normal among friends, family, and even friendly colleagues. Leaving it off is not rude, but adding it signals closeness.
Family names almost always take it
Family words rarely appear bare. You will hear maman joon (mum dear), baba jan (dad dear), khale joon (auntie dear). Grandparents are often addressed as maman joon or agha joon as a name in itself. If you are meeting a partner's family, attaching joon to how you address them is one of the easiest ways to sound warm rather than stiff.
Joonam: my dear, and a warm "yes?"
Joonam جونم adds "my", so it means my dear. It doubles as the standard warm reply when someone calls your name. Someone says "Sara?", and Sara answers "joonam?", literally "my life?", functionally a soft "yes, what is it?". It also appears as "pardon?" when you did not catch what someone said.
Swearing by a life: joone man
Because joon means life, it appears in oaths and disbelief. Joone man? means "really? you swear?", literally "on my life?". Be joone madaram, on my mother's life, is a strong way to insist you are telling the truth. These are everyday spoken patterns, not dramatic ones.
Joon versus jan
They are the same word. Jan is the formal and written form and is also standard in Afghan Persian, while joon reflects the Tehran habit of shifting the long "a" sound to "oo" in casual speech, the same shift that turns nan (bread) into noon. Use either after a name; joon sounds a touch more casual and intimate.
One caution: joon on its own, said to a stranger, can sound flirtatious or cheeky, so keep it attached to names or inside set phrases until the register feels natural.
Start with names: add joon to the people you are closest to, answer with joonam when they call you, and the rest of the word's range will come with exposure.