Orthokeratology (Ortho-k) is the design and fitting of special gas permeable contact lenses that gently reshape the clear front surface of the eye (cornea) while you sleep, so you can see clearly without lenses during the day.

Orthokeratology is suitable for myopia (nearsightedness) which is caused by the cornea being too steep. Ortho-k corrects myopia by reshaping the cornea so light entering the eye is more accurately focused on the retina.

The corneal reshaping effect of orthokeratology is temporary — generally enough to eliminate your need for glasses or contacts for a day or so. Then you have to wear the ortho-k lenses again to maintain the proper corneal shape.
This is a great alternative for patients who want clear vision without surgery. More info here.