The Hebrew word translated “mercy seat” is Strong’s 3727 pronounced “kapporeth”. It is found 27 times in the OT. It is derived from the Hebrew, “kaphar”, which simply means “to cover”. Thus, “kipporeth” is “the cover” of the “ark” or “box of the covenant”. This “cover” is what we know as the “mercy seat”. Simply put it is the cover of the box. The first passage in the Old Testament “kapporeth” is found is Ex 25.
(1) And Yehovah spoke to Moshah saying, “Speak unto the children of Yisrael, that they bring me an offering. Of every man that giveth it willingly with his heart you shall take my offering…(8) And let them make me a “miqdash” (Strong’s H4720, separated or set apart place), that I may dwell among them. According to all that I show you, after the pattern of “mishkan” (H4908, my dwelling-place), and all the “keliy” (H3627, items), even so shall you make it. And they shall make a “arown” (ark, H827, box) of “shittah” wood (The shittah tree is called acacia tree. Also called the “thorn tree” http://en.wikipedia.org/wiki/Acacia. Its thorns were used for scourging. Examine Strong’s entry 7848 & 7850 to see this relevance. Why would God want this box specifically made out of the wood of a thorny tree? Could it be to prophesy of His eternal purpose in Yeshua ha Mashiach? Yes, of course!), two cubits and a half the length thereof, and cubit and half the width thereof, (the box is rectangular in shape) and a cubit and a half the height thereof. And thou shalt overlay it with pure gold, within and without shall thou overlay it, and thou shalt make the gold flow around it (flowed, not beaten)…(17) And thou shall make a “kapporeth” (covering) of pure gold. Two cubits and a half the length thereof, and a cubit and half the breadth thereof. And thou shalt make two “keruwbiym “ (the plural of keruwb) of gold, of beaten work shall thou make them, for the two ends of the “kapporeth”. And make one “keruwb” on the one end, and the other “keruwb” on the other end, on the “kapporeth” shall you make the “keruwbiym” on the two ends thereof. And the “keruwbiym” shall stretch forth their “kanaph” (“wings”, Strong’s 3671, can be translated skirt. When “kanaph” is translated “wings” there is usually another word that implies a creature with wings, such as Gen 1:21, “winged” fowls. There is no implication of a winged creature in this passage, thus, I would say that skirt or robe would be a better translation.) on high, covering the “kapporeth” with their “kanaph” (Kanaph is skirt or garment.) , and their faces look one to another (The cheruwbiym were not winged creatures, but they both lifted their skirt as high as to not obscure them from looking at each other. They await to receive Him who is to be raised from the dead. These things most certainly prophesy of the Mashiach. ) , toward the “kapporeth”(and not away) shall the faces of the “keruwbiym” be. And thou shalt put the “kapporeth” upon the “arown” (box) and in the “arown” (box) thou shalt put the “eduwth” (“Eduwth” interpreted means “testimony” or “witness”, Strong’s 5715) that I shall give thee. And there shall I meet with thee, and I will commune with thee from above the “kapporeth” upon the “box of the witness” from between the two “keruwbiym” of all things which I give thee (speaking to Moshah) in commandment to the children of Yisrael.” And so it is written about the “kapporeth” (called mercy seat) and the “arown” ( called ark, and is a box).
The first use of the Hebrew word “arown” in the OT is Gen 50:26, “So Yowceph died being an hundred and ten years old. And they emblamed him, and he was put into a “arown” (translated coffin, the box for the dead) in “Mitsrayim” (Egypt).”
The word “mercyseat” is found one time in the NT in Hebrew 9:5. The Greek word that translates “kapporeth” in the LXX and NT is “hilasterion” (Strongs G2435). It is used two times in the NT, Hebrew 9:5 and Rom 3:25, translated “propitiation”. “Hilasterion” is derived from the Strong’s G2433 which in turn is derived from G2436. In these words the sense of “mercy” can be seen and where eventually the word mercy seat was used to translate “kapporeth”. But the orginal Hebrew language has no such meaning of “kapporeth”.
FYI. The shittah tree called acacia remains relevant in our day and hour – http://www.todayshomeowner.com/choosing-durable-wood-for-a-garden-bench-and-outdoor-furniture/