filamentous

filamentous — adjective

  • filamentouspositive
  • more filamentouscomparative
  • most filamentoussuperlative

1. having a very thin, long shape like a fine thread or fiber, especially when desc

1.adjectiveC1
Definition

having a very thin, long shape like a fine thread or fiber, especially when describing something that can only be seen clearly under a microscope

Example

The biologist observed filamentous bacteria growing across the surface of the agar plate.

attributive use: filamentous bacteria

When mold grew on the bread, it formed a white filamentous mat that spread quickly.

Common collocations
  • filamentous bacteria
  • filamentous fungi
  • filamentous algae
  • filamentous structure
  • filamentous growth
Synonyms
  • threadlike

    less technical and more widely understood; works for everyday descriptions of thin, stringy shapes

  • fibrous

    suggests thicker, tougher strands rather than the delicate, fine-thread quality of filamentous

  • thready

    very close in meaning but much rarer in modern scientific usage

Usage Note

Common in scientific writing, particularly biology and microscopy. Typically used attributively before a noun (filamentous bacteria, filamentous fungi), though predicative use after a linking verb is also possible (the bacteria are filamentous).

Word family

Synonym discussion

Sense: adjective/1

Filamentous, threadlike, fibrous, thready, and fibrillar all describe thin, elongated shapes, but they differ in register and precision. Filamentous is the standard term in biology and microscopy for structures that are as thin as a filament — used for bacteria, fungi, algae, and cellular components. Threadlike is its everyday equivalent: any object that resembles a thread qualifies, from hair to string beans, without requiring a scientific context. Fibrous suggests thicker, coarser strands that form a tough network, like muscle fibres or plant stems, rather than the delicate single-strand quality of filamentous. Thready is a direct synonym but feels dated and is rarely used in modern lab reports. Fibrillar is the most technical, reserved for sub-microscopic structures such as collagen fibres in connective tissue. In a biology paper, filamentous is the safe default; in a general description, threadlike communicates more clearly to non-specialists.

threadlike ↗fibrousthreadyfibrillar

Common collocations

Nouns

Etymology

LatinEnglish

Derived from the Latin word filamentum, meaning 'a thread', which comes from the verb filare meaning 'to spin'. The English suffix -ous, meaning 'full of' or 'having the qualities of', was added to form filamentous in the early 1800s, primarily in natural sciences describing thread-like organisms and structures.