The phrase "single filament" refers to a single thread-like strand of a material, such as a thin wire or hair, that is not divided into multiple parts. Full definition
Monocryl is an absorbable, single filament suture with low tissue reactivity which dissolves slowly and loses strength. (sciencedaily.com)
They're simple, single filaments, found in dense forests and unrecognisable as feathers. (discovermagazine.com)
But he and others point out that hard evidence for this hypothesis has been lacking, largely because the single filaments found on these plant eaters lack the complexity of the protofeathers found on early meat eaters. (sciencemag.org)