Just to add to the (potential) confusion, this is another term used differently in UK gliding from UK power instruction. I don’t know about other countries.
In gliding we use side slip approaches to get rid of surplus height under some circumstances, in addition to airbrakes (or, for our USA cousins, spoilers). It is also used instead of crabbing sometimes, to cope with cross winds.
In either use, I have never heard it called anything but a side slip, but (some of?) the power people call it a forward slip.
We also sometimes use slipping turns, e.g. when much too high on final turn. This is an under-ruddered turn (not likely to cause a spin, which can result from an over-ruddered turn).
In side slips and slipping turns, the ASI is rendered inaccurate to the point of being useless, by the way.
Chris N.