[Macha]

As a margin of error.

"this tree was planted 200 years ago, +/- 10 years" -> the tree was planted between 190 and 210 years ago.

"this comet will next be seen in 100 years, +/- 10" -> The comet will visible between 90 and 110 years from now.

[wbercx]

Yeah, interesting. In The Netherlands it is commonly used to mean "more or less", or "approximately", unless it is used in formulas.

[davidbanham]

For “approximately 10 years” I would note it as ~10. Native Australian English speaker.

[ben174]

In English, "give or take".