From the definitions I've read online they all go something like this...
Sampling bias occurs when some members of a population are systematically more likely to be selected in a sample than others.
I'm going through a biometrics course at my university and while looking at the answer keys to previous exams, there are questions about hypothetical studies with regards to identifying bad sampling that have "bias" as an answer and don't seem to meet the definition in the course or else where.
For academic integrity reasons I won't post the exact questions, but I suppose an example of such would be...
A large portion of fish were able to escape from your fishing net because there was a small hole on the side of it. What is the obvious issue with this sampling?
How exactly would this be a "biased" sample if it isn't systemic?