'Underlying' is an adjective used to describe something which is not immediately obvious or apparent "She had an underlying condition which had not been diagnosed." "The underlying reason for her behaviour was identified by the psychologist."Something underlying may also describe something upon which something else rests or depends. "The underlying structure fell apart when they were working on the top level." "The underlying support of her colleagues made the research much easier to complete." You will often hear the phrase 'the underlying cause' of something, meaning the ultimate reason for something.