A ductless glandular organ at the base of the neck that produces lymphocytes and aids in producing immunity; atrophies with age WordNet 3.0 & Open English WordNet

The thymus is a specialized primary lymphoid organ of the immune system. Wikipedia

An organ Wikipedia Disambiguation

An organ of the immune system Wikipedia Disambiguation

Endocrine gland Wikidata

A ductless gland, consisting mainly of lymphatic tissue, located behind the top of the breastbone. It is most active during puberty, after which it shrinks in size. It plays an important role in the development of the immune system and produces lymphocytes. Wiktionary

Ductless gland located behind the breastbone. Wiktionary (translation)