Definition:

The so-called "small ball" was a golf ball approved for play by the R&A that was slightly smaller in size than the golf ball approved for play by the USGA. The USGA ruled the small ball illegal in the early 1930s. Golfers playing in competitions governed by the R&A had the option of using the small ball. It wasn't until 1990 that the R&A and USGA finally standardized golf ball size for all golfers everywhere.

See "British Ball" for more details about the small ball and its history.