An invisible zipper is a zipper with very fine teeth. The way an invisible zipper is sewn in to a garment, makes the zipper invisible and hidden in a seam. The only part of the zipper that is left visible is the pull tab and that tends to be smaller or less obtrusive than other types of zippers. No top stitching is needed on an invisible zipper.
An invisible zipper is commonly used in formal wear type of clothing. An invisible zipper in a gown will not distract from the lines or flow of the gown.
Invisible zippers are not available in as many length variations as as all purpose zippers. They are also not available in as many colors but because they are hidden in the seam, a color as close as possible is fine.
Special sewing machine feet are needed to sew in an invisible zipper. These feet are available where invisible zippers are sold. These feet hold the zipper coil so that the stitching is where it needs to be to keep the zipper invisible.
Unlike a regular zipper, when sewing in an invisible zipper, the seam is sewn after the invisible zipper is sewn. Be sure to read the directions before sewing the zipper area.