Two Fractions are said to be adjacent if their difference has a unit Numerator. For example, 1/3 and 1/4 are adjacent since , but and are not since . Adjacent fractions can be adjacent in a Farey Sequence.
See also Farey Sequence, Ford Circle, Fraction, Numerator
References
Pickover, C. A. Keys to Infinity. New York: Wiley, p. 119, 1995.