I have a synthesizer and I have a stereo sound that needs to have reduced stereo spread. I only can use an onboard synthesizer effect for that, and I was wondering if there is an effect known by mixing/mastering to use for this?
Yes, there are plenty of ways to do this when mixing. Normally each of the left and right signals have their own channel in a mixer and can both be panned center or wide left/right or anywhere in between. There are also stereo imaging plugins that can narrow or widen the spread of a stereo signal and change the "center" of the signal. As Stelios mentions, it is possible to run a stereo signal through a mid-side processor and change the stereo spread that way.
It is possible for certain kinds of compression to change the apparent stereo width also, with lat/vert mode on a record (vinyl) mastering compressor being a famous example.