1. a set of equations in two or more variables for which there are values that can satisfy all the equations simultaneously;

The Collaborative International Dictionary of English v.0.48:

Simultaneous \Si`mul*ta"ne*ous\, a. [LL. simultim at the same time, fr. L. simul. See Simulate.] Existing, happening, or done, at the same time; as, simultaneous events. -- Si`mul*ta"ne*ous*ly, adv. -- Si`mul*ta"ne*ous*ness, n. [1913 Webster] Simultaneous equations (Alg.), two or more equations in which the values of the unknown quantities entering them are the same at the same time in both or in all. [1913 Webster]WordNet (r) 3.0 (2006):

simultaneous equations n 1: a set of equations in two or more variables for which there are values that can satisfy all the equations simultaneously