- 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.
- 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.
- (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.