What is 'Spectral Domain'? In a paper which deals with optical coherence tomography they say they are dealing in the spectral domain. What exactly does that mean? 
 A: Generically speaking, optics makes a distinction between the time domain, which deals with the explicit time dependence $E(t)$ of the electric field of the light, and the frequency domain, in which the core emphasis is shifted to the Fourier transform of the field,
$$
\tilde E(\omega) = \frac{1}{\sqrt{2\pi}} \int E(t) e^{i\omega t}\mathrm d t.
$$
The terms 'spectral domain' and 'frequency domain' are essentially interchangeable in this context.
In optical coherence tomography, you have a sample that you want to interrogate, and you examine the back-scattered radiation by manipulating it coherently and comparing it against a stable reference. Some of these manipulations are best seen as changes (such as delays) in the time domain, while other examples are best analyzed through the prism of $\tilde E(\omega)$; the latter are known as Fourier-domain / frequency-domain / spectral-domain techniques. (Unfortunately, it's hard to go into more detail, particularly for such a broad question, since the field of OCT is so broad and heterogeneous.)
