Brand kit
The single place a brand's operating decisions are recorded: colour palette with usage rules, typeface pairing and scale, logo direction and clear space, written voice, and the strategy underneath them. It exists so that a new piece of work nobody anticipated can still be made the same way every time.
Brand kit is not Brand guidelines.
Guidelines are a document written to tell other people what they may not do, and they are usually long, PDF-shaped and rarely opened. A brand kit is the working set you reach for while making something, so it is judged on whether it is at hand at the moment of a decision rather than on how thorough it reads.
In practice
Most people meet the phrase as a folder: a logo file, a few hex codes, maybe a font. That folder is useful and it is not a brand kit. The difference is that a folder stores outputs while a kit stores decisions, including the reasoning that governs when each output is used. A hex code tells you a colour exists. A kit tells you that this particular colour appears on one call to action per screen and nowhere else, which is the part that actually keeps two pages looking related.
Five parts do the work: palette, typography, logo direction, voice and strategy, in that order of visibility and reverse order of importance. Strategy is what makes the other four non-arbitrary, and voice is the one most kits skip despite writing being the surface a brand produces most of. We have written the long version of what belongs in each, and why the order matters, in the essay on what a brand kit actually is. This entry is the reference; that is the treatment.
The practical test of a kit is not whether it looks impressive but whether it answers a question it did not anticipate. A folder of assets covers the cases somebody thought of. A kit covers the case that turns up on a Thursday afternoon, when you need a background colour for a testimonial card nobody has ever made before and the decision has to be made in ten seconds. If the kit cannot answer that, it will be overruled by whatever is nearest, and every override becomes a variant that nobody knows exists.
Two habits keep a kit alive. First, keep it small enough that somebody will actually read it: a kit nobody opens loses to guesswork every time. Second, treat each unanticipated decision as a new rule. When you hit a case the kit did not cover, whatever you land on becomes the entry that stops the same question being answered differently next month. A kit is best understood as the accumulated memory of every decision you have already made once.