Where does the 'T' in T cell come from?

T cells are named after their site of development: The thymus gland 

The thymus is a primary lymphoid organ located between the lungs. After generation in the bone marrow, T cells migrate to the thymus and interact with surrounding cells to further develop and ensure that they will only recognise foreign antigen. This process is called thymic selection. When this process occurs incorrectly T cells that are reactive towards ‘self’ antigen can escape into the body causing autoimmune disease. For example, in Type I diabetes, insulin-producing pancreas cells are killed off by T cells that have escaped proper thymic selection