A serger or overlock is a type of sewing machine that can both cut fabric and make a stretch lockstitch.
Sergers use multiple threads and needles to not only make the seam, but also lock the fabric. They are very often used with knit or other stretchy fabrics.
The term overlock is more common in Europe for a serger. However, I prefer to use serger as it avoids confusion between the very similar named overlock and coverlock machines.
Further confusion is added by the fact that some sewing machines have an overlock stitch that mimics the role of a serger.
To steer clear of all this, I use serger to refer to the dedicated machine.