A strobogrammatic number reads the same when rotated 180°. The allowed digits are 0, 1, 8 (which are self-symmetric) and the pair 6/9 (which swap). Other digits don't have a clean rotational counterpart.
The first strobogrammatic numbers: 0, 1, 8, 11, 69, 88, 96, 101, 111, 181, 609, 619, 689, 808, 818, 888, 906, 916, 986, 1001.
The set depends on font — most digital displays render 1, 0, and 8 symmetrically and 6/9 as rotations of each other. Strobogrammatic primes (prime AND strobogrammatic) are rare: 11, 101, 181, 619, 16091, 18181, 19861, 61819, 116911, …