An undulating number has digits that bounce between two distinct values in an a-b-a-b pattern: 121, 484, 1717, 69696, 80808. The name evokes a wave. We require at least three digits and two different digits, so repdigits like 111 don't count.
The two-digit "seed" can be anything, giving families like 1010…, 1212…, 9090…. Undulating primes (an undulating number that's also prime, like 16361 or 1919191919191919191) are studied as a curiosity; it's known, for instance, that no undulating number with more than a handful of specific forms can be a perfect square.