@nigel there is; you select the bottom option from the privacy settings while writing your post (click on the globe icon, and select the "Direct" option).
This is not truly private, as the instance administrators of all of the instances involved (your instance, the recepients' instances) can still see it. But it at least (most likely) won't be public that way.
This Mastodon instance is for people interested in technology. Discussions aren't limited to technology, because tech folks shouldn't be limited to technology either!