Definitions of underground
  1. adverb
    beneath the surface of the earth
    “water flowing underground
  2. adverb
    in or into hiding or secret operation
    “the organization was driven underground
  3. adjective
    under the level of the ground
    underground caverns”
    synonyms: belowground
    beneath the surface
  4. adjective
    conducted with or marked by hidden aims or methods
    underground resistance”
    synonyms: clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover
    secret or hidden; not openly practiced or engaged in or shown or avowed
  5. noun
    an electric railway operating below the surface of the ground (usually in a city)
    synonyms: metro, subway, subway system, tube
    see moresee less
    type of:
    railroad, railroad line, railway, railway line, railway system
    line that is the commercial organization responsible for operating a system of transportation for trains that pull passengers or freight
  6. noun
    a secret group organized to overthrow a government or occupation force
    synonyms: resistance
    see moresee less
    the French underground that fought against the German occupation in World War II
    type of:
    revolutionary group
    a political unit organized to promote revolution
Word Family

Test prep from the experts

Boost your test score with programs developed by’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