Being unrestricted, scientific theories cannot be verified by any possible accumulation of observational evidence. The formation of hypothesis is a creative process of the imagination and is not a passive reaction to observed regularities. A scientific test consists in a persevering search for negative, falsifying instances. If a hypothesis survives continuing and serious attempts to falsify it, then it has ``proved its mettle'' and can be provisionally accepted, but it can never be established conclusively. Later corroboration generates a series of hypothesis into a scientific theory.
Thus, the core element of a scientific hypothesis is that it must be capability of being proven false. For example, the hypothesis that ``atoms move because they are pushed by small, invisible, immaterial demons'' is pseudo-science since the existence of the demons cannot be proven false (i.e. cannot be tested at all).
Excerpt from the Encyclopedia Britannica without permission.