1. The process by which one state changes into another. Anything which is not static evolves over time. The word evolution specifically excludes any connotation of improvement or progress, so even 'negative' changes are evolution. In biology, this definition is often observed in terms of the change of gene frequencies in natural populations. There is no controversy about this definition.
2. The theory that species give rise to other species. This process of speciation can be accomplished in a number of ways, all of which must result in two (or more) populations becoming genetically isolated. This can be accomplished physically (allopatry and peripatry) or through division of patch types or through behavioral mechanisms (sympatry). This has been observed at low taxonomic levels. For instance, wheat is a modern species resulting from 2 instances of non-disjunction and one hybridization event that all occurred within the last 5000 years. The original plant has been identified from seeds from tombs in Egypt.
The creation of new higher level taxa is often referred to as macroevolution by non-scientists, but this usage is incorrect. The only difference between the origins of higher taxa and species is time. Today's species will eventually give rise to genera as they continue to diverge from their sister-taxa and continue to speciate themselves. All higher taxa were once just a species. Macroevolution properly refers to selection that operates above the species level, such as the selective pressure across the K-T boundary on gastropods which favored genera (but not species) with wide geographic ranges.
I'll note that theory is the most certain science ever is. In this case, it means that over the last century and a half, plus the observations from before the theory's proposal, no evidence has been found that contradicts the theory of evolution. By the same token, gravity is a theory, and thermodynamics is a theory. 'Law' is just scientific shorthand for a theory that no one thinks will ever be disproved, and may soon be applied to evolution.
1. The reaction was in a critical phase of its evolution.
2. The fossil sequence of horses is an excellent example of evolution