Schema

A schema is the blueprint of a dataset: the list of columns, each column's name and data type, and the rules they must obey (this column can't be empty, that one must be a positive number). It says what well-formed data looks like before any rows arrive.

Schemas catch problems early and make data trustworthy: if incoming data violates the schema — a text value where a date belongs — it can be rejected at the door instead of corrupting your analysis. Even an informal, written-down schema turns vague assumptions about a table into checkable rules.