Scientific Theory
A scientific theory is a well-tested, broad explanation of natural phenomena supported by a large body of evidence; it is not a guess.
Definition
A scientific theory is a big explanation that has been tested many times and always held up, tying many different observations together with one idea. It is a broad, well-substantiated explanation of some aspect of the natural world, built from many confirmed hypotheses, repeated experiments, and observations by many scientists, and it makes predictions that can be tested; a hypothesis is a single testable idea, while a theory is the framework that many tested hypotheses fit into. Theories unify observations and laws, generate novel predictions, and have survived repeated attempts at falsification across independent lines of evidence. They are the most reliable form of scientific knowledge and still provisional, open to refinement, to bounding within a domain, or to replacement when new evidence demands it.
Example
The theory that all matter is made of tiny moving particles explains why salt dissolves, why ice melts, and why a balloon expands in the sun. The kinetic theory of matter, which says particles move faster at higher temperatures, explains the lab result that salt dissolves in $14\ \text{s}$ at $50^\circ\text{C}$ but $48\ \text{s}$ at $10^\circ\text{C}$, and also explains thermal expansion, gas pressure, and evaporation. It predicts that dissolving rate should rise with temperature in a specific way, so a lab finding that dissolving time roughly halves for each $20^\circ\text{C}$ increase is consistent with it, while a lab finding no temperature effect would be a serious anomaly demanding explanation.
Key Insight
In everyday talk "theory" means a guess; in science it means the opposite, an explanation so well supported that it would take strong new evidence to change it. A theory never graduates into a law, because theories explain and laws describe, and both stay open to revision. Evolution, plate tectonics, and atomic theory are called theories not because they are uncertain but because they explain, and explanation is the highest goal of science.