a seer: a prophet who receives God's message through visions
The word is built from a verb meaning "to see" (chazah), and it names exactly that: one who sees. A seer is a prophet whose defining mark is that God's message reached him as something perceived, a vision, which he then passed on. The title describes the channel the revelation came through more than the act of speaking it out loud.
Many seers appear as named figures attached to a royal court. Gad is called "David's seer," and the record of Israel's kings repeatedly cites the writings of court seers such as Iddo and Asaph. In these settings the seer is a recognized office, a trusted adviser who brought God's word to the king.
The same word also covers prophets as a class, and it can carry a sharp edge. A priest tells the prophet Amos to take his seeing elsewhere; the people of Isaiah's day order the seers to stop seeing true things and to tell them pleasant lies instead; and Micah pictures the seers put to shame when no vision comes. Whether honored or resented, the seer is the person expected to see what God is showing.