A steward is the operating half of ownership. They write and maintain the definitions in the business glossary, translate policy into rules, triage quality issues, and are the first call when a number looks wrong. They are chosen for expertise, not seniority — usually the person the rest of the team was already asking, informally, before this role existed. The best stewards were already doing most of the job unpaid and undocumented.
In practice. Stewardship is a named percentage of a real job, not a title added to a full workload. Ten to twenty percent, protected, with the owner's backing, is the difference between a steward network that works and one that quietly stops meeting after four months.
Where it goes wrong. Stewards are appointed in bulk, given a template and no time, and asked to document everything. Nothing gets documented, and the program concludes that the business is not engaged. Steward capacity is the single most common thing to be under-resourced in a governance program, and the easiest to fix.