Introduced by Meyer Fischer 1971, as a problem of 1D against
which the optimal 1NFA is no smaller than the optimal 1DFA. (The
original definition allowed arbitrarily long instances, but this is not essential in this context.)
The name “length” is suggested by this site. huh?
See iterated length for the variant where the length must be a multiple of $h$,
and long length for the variant where the length must be $2^h$.