A schema is a plan or an outline that is also a diagram. It is pronounced (skee ma). The plural may be made by either schemas or schemata. Note, that schemata, while uncommon, is often misused as a singular form, but it is in fact plural.
There are many derivatives of schema, including schematic. Schematic, pronounced (skee maa tick), may be used as an adjective or noun. Because schema dates from around 1700 and schematic from the 1930s, our guess is that the noun form of schematic came from the pervasive misuse of the adjective. That is pure speculation on our part, based on how words tend to change over time.
The adverb form is schematically, and it is usually used when referring to the schematic or plan of events.
Scheme comes from the same root, and can be used to mean a plan or outline, but the more common definition is a plan in which to gain something by deception or thievery.
Pearson’s schema won the day not only because of its upscale presentation and strong media support but because it came with a prescription, a cluster of linked programs to change behaviour: if parents failed to send their children to school their welfare income would be quarantined after review by a local panel, the Family Responsibilities Commission. [The Australian]
Multiculturalism was always far more prevalent in Europe than in America, perhaps because Europeans have always been more ready to accept new political schemas. [The Mercury]
Apparently this schematic was leaked from Foxconn, Apple’s primary assembler of their products, so perhaps there could be some truth to these claims. [Uber Gizmo]
Commercial building in Liverpool requires electrical and plumbing schematic drawings preparing following planning permission approval. [Industry Today]
Schematically we did not follow what we were supposed to do. [Pantagraph]