Inflation is the idea - first proposed by Alan Guth (1981) - that the nascent universe passed through a phase of exponential expansion that was driven by a negative vacuum energy density (positive vacuum pressure). As a direct consequence of this expansion, all of the observable universe is posited to have originated in a small, initially causally-connected region. Quantum fluctuations in this microscopic region, magnified to cosmic size, then became the seeds for the growth of structure in the universe (See Galaxy Formation and Evolution).
Inflation resolves several outstanding problems in standard Big Bang cosmology. Among these are the observed flatness of the universe (the flatness problem), its extraordinary homogeneity on large (non-causally-connected) scales (the horizon problem), and its lack of any observed topological defects. Topological defects (kinks in a massless scalar field) such as monopoles, cosmic strings, and domain walls, are predicted by many Grand Unified Theories.
Predictions of the standard model of inflation include geometrical flatness of the universe to high precision and scale invariance of the fluctuations in the cosmic microwave background. There are also consequences for high-energy particle physics near or at the GUT scale. The exact nature of the particle or field that generates the vacuum energy density for inflation (the "inflaton") remains a mystery.