An abnormal or unwanted outgrowth or projection on the body or a surface; something that grows excessively beyond what is normal.
From Latin excrescentia (an outgrowth), from excrescere (to grow out); originally referred to any external growth but came to mean specifically abnormal or undesirable ones.
A wart, a cyst, or even a mole—these are all excrescences in the medical sense, but the word also applies metaphorically to anything 'growing out' where it shouldn't, like an awkward subplot in a story.
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