Patient safety starts with a simple rule: first, do no harm. This chapter defines the core terms that anchor safety work-preventable adverse event, near miss, sentinel event, medication error (any avoidable failure in the medication process), and adverse drug reaction (a noxious, unintended response at normal doses). It clarifies their relationships: medication errors may lead to adverse events, but not all errors cause harm; adverse drug reactions can occur without any error; and some adverse events arise outside the medication domain. We map common sources of error across people, processes, and systems: communication and handoff failures, cognitive biases and diagnostic delay, unsafe workarounds, workload and fatigue, equipment and device issues, prescribing/dispensing/administration mistakes, and lapses in infection prevention.