A single, consensus definition of a soliton is difficult to find.  ascribe three properties to solitons:
# They are of permanent form;
# They are localized within a region;
# They can interact with other solitons, and emerge from the collision unchanged, except for a phase shift.
More formal definitions exist, but they require substantial mathematics. Moreover, some scientists use the term soliton for phenomena that do not quite have these three properties (for instance, the light bullets of nonlinear optics are often called solitons despite losing energy during interaction).