A number with a continued fraction whose terms are the values of one or more Polynomials evaluated on consecutive Integers and then interleaved. This property is preserved by Möbius Transformations (Beeler et al. 1972, p. 44).
References
Beeler, M.; Gosper, R. W.; and Schroeppel, R. HAKMEM. Cambridge, MA: MIT
Artificial Intelligence Laboratory, Memo AIM-239, Feb. 1972.