* Your uncle: "Un oncle"
* An uncle in general: "Un oncle"
Examples:
* "J'ai un oncle qui habite à Paris." (I have an uncle who lives in Paris.)
* "Le petit garçon a vu un oncle à la fête." (The little boy saw an uncle at the party.)
You can also use "mon oncle" if you are talking about your own uncle, but "un oncle" is more general.