Definition of pileus in US English:


nounPlural pilei

  • The cap of a mushroom or toadstool.

    • ‘Their compact pilei - tight-fitting in infant plants, robust and appetizingly domed in ripe ones - have a smooth (not lamellate) undersurface and a neat, strong stem.’
    • ‘The development is initiated as a hyphal knot of 0.2 mm or less in diameter, which rapidly develops into a tiny primordium containing the rudimentary tissues of the future pileus and stipe.’
    • ‘CO2, levels above 1,000 ppm repress expansion of the pileus and stimulate stipe growth in a wide variety of commercially-grown species, leading to spindly mushrooms in high CO2 conditions.’


Mid 18th century: from Latin, literally ‘felt cap’.