Webster's English Dictionary

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thymus

thy.mus \'thi--m*s\ n [NL, fr. Gk thymos warty excrescence, thymus] : a 
   glandular structure of largely lymphoid tissue and uncertain function  that 
   is present in the young of most vertebrates typically in the upper anterior 
   chest or at the base of the neck and tends to disappear or become 
   rudimentary in the adult