A combination of pure starch (i.e. cornstarch, flour or arrowroot) and water or another liquid used to thinken liquids to make a sauce or gravy.  Slurries are excellent for last minute thickening and should be mixed to the consistency of heavy cream.  

A useful ratio of 1 Tbsp starch to 1 Tbsp water  will thicken  1 cup of liquid to a medium consistency.  Your slurry should be completely smooth and without lumps or clumps of starch.  Bring the liquid to be thickened to a simmer.  Quickly stir the slurry, and pour it into the simmering liquid in a slow, steady stream, stirring the liquid continuously.  Once added, continue stirring and cooking the liquid as it simmers and thickens to the deisred consistency, about 1-2 minutes.