Often bandied about by skeptics when dismissing claims by proponents of alternative medicine, anti-vaccine, creationism, paranormalists etc, the word pseudoscience has a very valid place in our lexicon. To understand what we mean by it, we need first to understand what we mean by 'science'. Wikipedia defines it as "a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe" and the Cambridge dictionary defines it as: "(knowledge from) the careful study of the structure and behaviour of the physical world, especially by watching, measuring, and doing experiments, and the development of theories to describe the results of these activities". Layered on top of that is a language, method, process and system that has arisen and codified over the last few decades. Things like journal papers, peer review, a body of evidence, regulation measures, academic standards, experimental procedures, and many more that are built on the aims of science.
Built into the pursuit is scientific skepticism; the acceptance that the easiest people to fool are ourselves, all conclusions are tentative and that all models built from data must explain the data better and more comprehensively than others. Pseudoscientists basically have many of these trappings but fail to apply them consistently and honestly which then skews the results. The most common failure is by assuming the conclusion before the test. A good example is creationism. Though some creationists may believe they are doing science and will argue that they are, because they start off with the premise that the world is 'created', they do experiments and perform studies to find confirmatory evidence that rather than look at ways it can be disproved. Many alt-med proponents will cherry pick and point to studies that seem to confirm their conclusion whilst all the while ignoring all the other evidence that contradicts it.
Pseudoscientists will often make unfalsifiable claims and point to studies or tests that are, at best, consistent with their conclusion, and because no test could contradict it, will say this supports their proposal. Just because something has the illusion of science, it does not mean they are actually doing science.