It is my understanding that a prophet is one who speaks for another.
According to the Strongs Concordance the word in the Old Testament is nabiy
5029 nbiy' neb-ee' (Aramaic) corresponding to 5030; a prophet:--prophet.
5030 nabiy' naw-bee' from 5012; a prophet or (generally) inspired man:--prophecy, that prophesy, prophet. 5031 nbiy'ah neb-ee-yaw' feminine of 5030; a prophetess or (generally) inspired woman; by implication, a poetess; by association a prophet's wife:--prophetess.
I have also heard or read somewhere that it can also mean "to bubble up" describing how a person speaks as the Holy Spirit within him/her bubbles up
In the New Testament the word is prophetes
4394. propheteia prof-ay-ti'-ah from 4396 ("prophecy"); prediction (scriptural or other):--prophecy, prophesying.
4395. propheteuo prof-ate-yoo'-o from 4396; to foretell events, divine, speak under inspiration, exercise the prophetic office:--prophesy.
4396. prophetes prof-ay'-tace from a compound of 4253 and 5346; a foreteller ("prophet"); by analogy, an inspired speaker; by extension, a poet:--prophet.
4397. prophetikos prof-ay-tik-os' from 4396; pertaining to a foreteller ("prophetic"):--of prophecy, of the prophets. 4398. prophetis prof-ay'-tis feminine of 4396; a female foreteller or an inspired woman:--prophetess. 5578. pseudoprophetes psyoo-dop-rof-ay'-tace from 5571 and 4396; a spurious prophet, i.e. pretended foreteller or religious impostor:--false prophet.
I have heard or read somewhere that the word prophetes can mean "one who speaks for another." In my Apostolic Bible Polyglott (which is a Greek/English interlinear Bible) with Strongs numbers the definition for prophetes is "declarer." In that Bible,
Exodus 7:1 reads: "And the Lord said to Moses, saying, Behold I have made you as a god to Pharaoh, and Aaron your brother will be your prophet." The Greek word translated prophet there is prophetes (no. 4396). As Aaron was the spokesperson for Moses, a prophet (prophetes) is a spokesperson for God --or a prophet is one who speaks for God.
Another Old Testament word for prophet was "seer" (reference
1 Samuel 9:6-9 where Samuel is called "a man of God" and also a "seer" with the explanation that the person usually called a prophet at the time this book was written used to be called a seer."
The word seer in the Old Testament is Strong's number 7300 or 2374
7200 ra'ah raw-aw' a primitive root; to see, literally or figuratively (in numerous applications, direct and implied, transitive, intransitive and causative):--advise self, appear, approve, behold, X certainly, consider, discern, (make to) enjoy, have experience, gaze, take heed, X indeed, X joyfully, lo, look (on, one another, one on another, one upon another, out, up, upon), mark, meet, X be near, perceive, present, provide, regard, (have) respect, (fore-, cause to, let) see(-r, -m, one another), shew (self), X sight of others, (e-)spy, stare, X surely, X think, view, visions.
or
2374 chozeh kho-zeh' active participle of 2372; a beholder in vision; also a compact (as looked upon with approval):--agreement, prophet, see that, seer, (star-)gazer.
It is my understanding that the term "seer" meant that a prophet had spiritual discernment or could see things from more than just a human viewpoint or could actually see visions.
Note: These quotes from Strongs are taken from
http://www.eliyah.com/lexicon.html
I used the second option of entering the Strong's number and choosing either Hebrew or Greek in Strong's Lexicon.
I do not read Hebrew or Greek so I am at the mercy of the authors of the Strong's Concordance.
Years ago there were three main concordances that I heard of: Cruden's, Strong's, and Young's. Someone said that Cruden's was for the crude, Young's was for the young, and Strong's was for the Strong. That was in the day before the internet when we had to look at paper copies of Bibles, commentaries, concordances, etc.