I apologize if this is too basic for MO.

I have an embarrassing admission to make: I don't know the *actual* definition of the discrete/continuous spectrum of a reductive group $G/\mathbb{Q}$ (in the context of automorphic forms if there is some ambiguity I'm unaware of).In particular, I know what the discrete series representations are for various groups $G$ (e.g. $\text{GL}_2$), and I assume that the discrete spectrum is made up of the discrete series (pluse extra! see the comments below), but I don't know how to define them in a general, 'elegant' way.

I have the, possibly wildly incorrect, impression that the discrete spectrum of $G$ is the largest semisimple $(\mathfrak{g},K)\times G(\mathbb{A}^\infty)$-subrepresentation of $L^2(G(\mathbb{Q})\backslash G(\mathbb{A}))$ and that the continuous spectrum is the orthogonal complement of this which should be some sort of 'direct integral' over continuous parameters. But, to be honest, in the searching that I've done I haven't seen this explicitly stated.

So, with this being said, I have the following three basic questions:

Question 1:What is the rigorous, 'elegant' definition of the discrete and continuous spectrum of a reductive group $G/\mathbb{Q}$?

Question 2:Why are these called 'discrete' and 'continuous'?

Question 3:Can we explicitly describe the discrete and/or continuous spectrum in any reasonable way for general $G$? If not, which $G$ can we describe it for?

**NB:** I am a relative neophyte, so I would appreciate if any answer could be in as simple language as possible.

Thanks!

**EDIT:** There's also the following 'bonus question' if anyone feels up to it:

Question 4:How do Eiseinstein representations fit into this? Why do they appear in both the continuous and discrete spectrum?