A finger wave is a method of setting hair

Hair

Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

into waves (curls) that was popular in the 1920s and 1930s and in the late 1990s in North America and Europe. The process involves pinching the hair between the fingers and combing the hair in alternating directions to make a wave shape. A lotion was applied to the hair to help it retain its shape. According to "Techniques of the 1920's and 1930's":

Finger waves were developed in the 1920's to add style to, and soften the hard appearance of, the bobbed hairstyles that became very popular during the flapper period.

Many Hollywood movie stars wore the latest finger waves which contributed to the popularity and evolution of this style.

FINGER WAVING is the shaping or moulding of the hair while wet into "s"-shaped curved undulations with the fingers and comb. These waves when dried without being disturbed will fall into beautiful deep waves. Finger waving differs from marcel waving in that there are no heated irons used on the hair. Not only naturally curly or permanently waved hair can be finger waved, but it is equally successful on straight hair.

Finger waves are similar to the Marcel Wave in appearance and are hence easily confused. The principal difference is that the Marcel Wave (sometimes spelt Marcelle and originally known as "Undulation Marcel") is achieved by means of heated curling irons, and are named after Francois Marcel, 19th century French hairdresser who invented the process in 1872. It revolutionized the art of hairdressing all over the world and remained in vogue for over fifty years, making a fortune for Mr Marcel. The other principal difference between the two different type of wave is that the Marcel Wave is more permanent than finger waves due to the use of a hot curling iron.