What are the definitions of symbols and samples related to PCM?
Quantization and encoding are largely independent. "Symbols" is another word for "pulses", and the line encoding can also play a role in how the information is transmitted.

Say you quantize one sample to $256 = 2^8$ levels, or 8 bits/sample. In order to transmit those 8 bits, you can, among other options:


use binary encoding, which requires transmitting eight symbols, each with amplitude in  the set $\lbrace +, - \rbrace$, where $+$ and $-$ correspond to some predefined voltage levels.
Use ternary encoding with AMI (alternate mark inversion), which also requires eight symbols, but now with amplitudes in the set $\lbrace +, -, 0 \rbrace$.
Use quaternary (or 2B1Q) encoding, which requires four symbols.


Other examples may include Manchester encoding, CMI, block line codes.... but in all cases, one symbol correponds to one transmitted pulse.