What is the actual meaning of "denpa"? After watching a lot of "Ryuusei no Rockman", I've come to know the word "denpa" as meaning "wave", or "EM wave". Though I have heard it used in Katekyo Hitman Reborn as well. Is the rough translation of "wave" accurate?
Japanese is a highly contextual language. Often times common words have uncommon alternate meanings. It's best if you cite examples for your Reborn context.

In the case of Megaman StarForce, it is what it literally means. 電波 literally means "electromagnetic/radio waves." The former character 電 means electricity. The latter character 波, means wave, as in billows upon some medium.

電波 has some slang usage as well; by itself it can be used to describe nonsense or something pointless, in other words, noise.

電波系 (denpa-kei) can be used to describe someone crazy, typically one with irregular or wild fantasies or delusions, such as hearing voices. This may have some bearing on your Reborn question, but without sufficient context, it's hard to tell.