If something is full of tiny holes or openings, you can describe it as porous. A sponge is porous, and if the border between countries is open for anyone to cross easily, it too can be called porous.

You can see the word pore — meaning "a tiny opening" — in porous. When potters make a mug, they use special glazes to seal the porous clay, which otherwise would absorb the liquid you put in the mug. This meaning has expanded so porous can describe any barrier that allows easy passage in and out, like the porous border between two countries that allows residents to move easily between them.

Definitions of porous
  1. adjective
    full of pores or vessels or holes
    synonyms: poriferous
    forming a continuous series of pores
    see moresee less
    not porous; especially not having vessels that appear as pores
  2. adjective
    allowing passage in and out
    “our unfenced and largely unpoliced border inevitably has been very porous
    synonyms: holey
    permitting the unwanted passage of fluids or gases
  3. adjective
    able to absorb fluids
    “the partly porous walls of our digestive system”
    “compacting the soil to make it less porous
    allowing fluids or gases to pass or diffuse through
Word Family

Test prep from the experts

Boost your test score with programs developed by Vocabulary.com’s experts.

  • Proven methods: Learn faster, remember longer with our scientific approach.
  • Personalized plan: We customize your experience to maximize your learning.
  • Strategic studying: Focus on the words that are most crucial for success.


  • Number of words: 500+
  • Duration: 8 weeks or less
  • Time: 1 hour / week


  • Number of words: 500+
  • Duration: 10 weeks or less
  • Time: 1 hour / week


  • Number of words: 700+
  • Duration: 10 weeks
  • Time: 1 hour / week