Realism
Realism is the claim that some class of things exists independently of us: the tree stands in the yard whether or not anyone looks, and its standing there is not made true by our thinking, our language, or our best theories.[1] The claim has two parts worth keeping apart — an existence claim (there are such things) and an independence claim (their nature doesn't depend on our minds). Most disputes are about the second.
Realism is always realism about something, and the arguments differ by subject. Realism about the external world is the everyday case, and its opponent is idealism. Scientific realism says our best theories are approximately true of things we cannot observe — that electrons are real and not just bookkeeping — and its opponents accept the predictions while declining the ontology.[2] Realism about universals says properties like redness exist in their own right, and is opposed by nominalism. Someone can consistently be a realist about tables and an anti-realist about numbers, morals, or unobservable particles.
For scientific realism, the strongest case is the no-miracles argument: if our theories were not at least approximately true, the fact that they keep making successful novel predictions would be an unexplained coincidence.
Against it, the pessimistic induction runs the history the other way. Theories once as successful as ours — phlogiston, the luminiferous ether, caloric — turned out to name nothing at all. If past success didn't secure truth then, why would present success secure it now? Realists usually reply by retreating to structural realism: what survives theory change is the mathematical structure, not the objects the old theory said were doing the work.