What is a fibril?

Fibril is a fine fiber approximately 1 nm in diameter, bundles of which may constitute a fiber. Any one of several threadlike fibers or maybe filaments which are major component areas of a cell or larger structure. Cellulose fibrils are the principal part of cell walls in plants

According to wikipedia - Fibril is a fine fiber, such as myofibril or neurofibril. Neurofilaments are about 10 nm in diameter.
Cytoplasmic fibrils are observed on the protoplasmic cylinders found in most spirochetal species, although no function of the cytoplasmic fibrils has been ascribed.