Using a sample of people who happen to be handy or easy to survey. May be OK in preliminary research, but not guaranteed to be representative of the population. Cf. random and purposive samples.
A sample for which cases are selected only on the basis of feasibility or ease of data collection. This type of sample is rarely useful in evaluation and is usually hazardous.
A type of non-probability sample in which the respondents have been selected because they are convenient for the researcher.