The original (1873/4) name for a game resembling tennis but played on lawns. Almost immediately it was perceived that calling it 'lawn tennis' would help attract the unlettered and those who had been attacked by sphygmomanometers in their formative years. (Actually the latter might have been coined a bit later—you wouldn't think it was possible to learn too much Ancient Greek, would you? But it was somehow contrived among the leisured classes of the late 1800s.)
Lawn tennis became so popular that tennis had to start huffily calling itself real tennis. Sadly, this left the trophy open for grabs, and lawn tennis is now usually called tennis.