The Collaborative International Dictionary of English v.0.48:

Interspace \In"ter*space`\, n. [L. interspatium. See Inter-,
   and Space.]
   Intervening space. --Bp. Hacket.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "interspace":
   caesura, clearance, discontinuity, dispart, distance between,
   double space, em space, en space, freeboard, hair space,
   half space, hiatus, interim, intermediate space, interruption,
   interstice, interval, jump, keep apart, lacuna, leap, leeway,
   make a space, margin, part, remove, room, separate, set apart,
   set at intervals, single space, space, space between, space out,
   time interval