A rictus is a frozen, fake smile. If the star of a play finds herself overcome by stage fright, she might forget her lines and stand, trembling, her mouth twisted into a rictus.
The word rictus most often describes a smile that doesn't convey delight or happiness — instead, it's a kind of horrified, involuntary grin. Your smile might freeze on your face in a rictus during a truly terrifying amusement park ride, or your discomfort as you watch your friend sing badly in a talent show might be reflected in your rictus of a grin. In Latin, rictus means "open mouth," from rict-, or "gaped."