Source : Webster's Revised Unabridged Dictionary (1913)

Fibril \Fi"bril\, n. [F. fibrille, dim. of fibre, L. fibra.]
   A small fiber; the branch of a fiber; a very slender thread;
   a fibrilla. --Cheyne.

Source : WordNet®

fibril
     n : a very slender natural or synthetic fiber [syn: {filament},
         {strand}]