noun \ˈthī-məs\
plural thy·mus·es also thy·mi \-ˌmī\

Definition of THYMUS

: a glandular structure of largely lymphoid tissue that functions in cell-mediated immunity by being the site where T cells develop, that is present in the young of most vertebrates typically in the upper anterior chest or at the base of the neck, that arises from the epithelium of one or more embryonic branchial clefts, and that tends to disappear or become rudimentary in the adult—called also thymus gland

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