lion: the big predatory cat, either a real animal or the picture of one standing for a kingdom's power
Nine of this word's ten appearances sit inside a single story. In Daniel 6 the king's officials trap him into signing a law he cannot undo, and the penalty for breaking it is a pit of lions. The phrase "the den of lions" comes back again and again as the trap closes, as Daniel is thrown in, as the king hurries out at first light, and finally as the accusers are thrown in themselves. The repetition is the point: the animals carry the whole weight of the threat, and the story is settled by whether their mouths open or stay shut.
The tenth appearance turns the animal into a picture. In Daniel 7 the first of four figures rising out of the sea is "like a lion" with the wings of an eagle. Kings across the ancient world carved lions onto their gates, their thrones, and their walls, so a lion standing for an empire needed no explanation to the first readers. Most scholars take that figure to be Babylon.
The word itself is doing nothing unusual in either place. It is simply the Aramaic name of the animal, matching the Hebrew word spelled almost the same way (אַרְיֵה) and the Greek word the New Testament uses (λέων). What changes between the two chapters is not the word but what the lions are: real ones pacing a pit, and then one seen in a dream.