"Lather" refers to the thick, foamy substance that is created when soap or shampoo is mixed with water. It is often used to clean and wash things, like our hands, face, or hair. Full definition
The stiff bristles are great for solid soaps and are especially ideal for whipping up a rich lather in a mug or shaving bowl. (businessinsider.com)
In the opening line of James Joyce's Ulysses, stately, plump Buck Mulligan bears «a bowl of lather on which a mirror and a razor lay crossed.» (firstthings.com)
In truth, even with those products designed to be used with a shaving brush, it's actually not necessary, but it will result in a better lather with less cream. (primandprep.com)