Tribadism refers to a sexual activity where you rub your genitals against some else's genitals, usually when you are both naked. This is also sometimes called scissoring.
The word was originally used to describe the same sexual activity of rubbing genitals together, but just when it was engaged in by two women. According to the Oxford English Dictionary, the word tribade was used as early as 1601 to describe a woman who engages in sexual activity with another woman.
Today tribadism is used for all combinations of genders, and is mostly distinguished from frottage by the fact that tribadism involves naked bodies, instead of clothed bodies, rubbing together. In neither activity is there any penetration.