This is the 9228th most frequent Russian word.
совершенно
"Completely" or "absolutely."
Here, "совершенно" is used as an adverb to intensify the adjective "невероятно," meaning "absolutely" or "completely."
Это совершенно невероятно.
This is absolutely incredible.
In this sentence, "совершенно" denotes the total extent of the verb "забыл," meaning "completely."
Я совершенно забыл об этом.
I completely forgot about it.
In the last example, "совершенно" modifies the adjective "другой," signifying a full extent of change, translated as "completely."