Bisexual is often used to describe attraction to men or women (trans OR cis), whilst pansexual's definition is below. The reason they exist separately is because pansexual is a more widespread term, while bi is, by definition, attracted to men in women. The term bisexual is derived from the Latin word 'bi', meaning two. The attached image further proves this. Bisexual may sound like it means the same as pansexual, but is by definition, different and less of an open term.