Difference between Nyquist rate and Nyquist frequency?
Harry Nyquist invented/discovered/proved a lot of things; it can be hard to keep track of them all. The three most important for signal processing and communications are probably these:


If you sample a (real) signal $s(t)$ at $f_s&gt;2B$ samples per second, then $s(t)$ can be reconstructed from its samples, where $B$ is the bandwidth of $s(t)$. The lower bound $2B$ is often called the Nyquist rate.
If you sample a signal $s(t)$ at $f_s$ samples per second, then $f_N=f_s/2$ is frequently called the Nyquist frequency (also folding frequency). All frequencies in $s(t)$ higher than $f_N$ will be aliased back to a frequency between 0 and $f_N$.
Assume you have a baseband channel of bandwidth $B$ (that is, signals with bandwidth $B$ or less are not distorted by the channel). This channel allows transmission of a maximum of $2B$ orthogonal pulses per second. $2B$ in this context is also called the Nyquist rate.