• enPR: lī'kən, IPA: /ˈlaɪ.kn̩/
• (also, especially in the UK) IPA: /ˈlɪtʃ.n̩/

lichen

1. Any of many symbiotic organisms, being associations of algae and fungi, often found as white or yellow patch#Noun|patches on old wall#Noun|walls, etc.
   • 1894 May, Rudyard Kipling, “Lukannon”, in The Jungle Book, London; New York, N.Y.: Macmillan and Co., published June 1894, OCLC 752934375 ↗, page 122 ↗:

     The Beaches of Lukannon–the winter wheat so tall, / The dripping, crinkled lichens, and the sea-fog drenching all!

   • 1895, H. G. Wells, The Time Machine, ch XI

     It was the same rich green that one sees on forest moss or on the lichen in caves: plants which like these grow in a perpetual twilight.

   • 1915, John Muir, Travels in Alaska, ch V

     The nibble marks of the stone adze were still visible, though crusted over with scale lichens in most places.

2. (figurative) Something which gradually spread#Verb|spreads across something else, cause#Verb|causing damage#Noun|damage.

   Synonyms: cancer

   • 1912 January, Zane Grey, “Shadows on the Sage-slope”, in Riders of the Purple Sage: A Novel, New York, N.Y.; London: Harper & Brothers Publishers, OCLC 6868219 ↗, page 202 ↗:

     Meanwhile, abiding a day of judgment, she fought ceaselessly to deny the bitter drops in her cup, to tear back the slow, the intangibly slow growth of a hot, corrosive lichen eating into her heart.

• French: lichen
• German: Flechte
• Italian: lichene
• Portuguese: líquen
• Russian: лиша́йник
• Spanish: liquen