rel·a·tiv·is·tic 
(rĕl′ə-tĭ-vĭstĭk)
(rĕl′ə-tĭ-vĭstĭk)adj.
1. Of or relating to relativism.
2. Physics
a. Of, relating to, or resulting from speeds approaching the speed of light: relativistic increase in mass.
b. Having to do with or based on the theory of relativity: relativistic mechanics.