When a sample is chosen because it is available to the researcher, it is known as

Convenience sampling selects units simply because they are easy to reach and available to the researcher.

It is a non-probability method, so results cannot be generalised to the whole population with confidence.

Probability sampling gives every unit a known, non-zero chance of selection, unlike this availability-driven pick.

Systematic sampling draws every kth element from an ordered list after a random start.

Objective sampling is not a recognised technical category in sampling theory.

Accidental or haphazard sampling are common alternative names for the same technique.