1,sth which you think so significant or beautiful , and can't discribe in words. eg. 微妙な色彩のバランス。splendid colorful balance. 2,sth which so difficult and complexity,and can't say it clearly. eg.両国の関係は微妙な段階になる。the relationship between two countries reaches to a complicated stage. and one of my japanese teacher told me that it can be used on eating sth.you feel it so good, and can't discribe it in words.