Brand positioning
The deliberate choice of what a brand stands for, for whom, and against which alternatives, expressed clearly enough that the choice excludes things. Good positioning names a specific audience, the problem it solves better than the obvious substitutes, and the reason that claim is believable. It governs every downstream decision about message and design.
Brand positioning is not Brand strategy.
Positioning is one decision: where you sit relative to the alternatives a buyer is actually weighing. Brand strategy is the wider plan for building and defending the brand, and positioning is its first and most constraining input. A strategy without a position is a schedule of activity.
In practice
Positioning is a decision about the space a brand occupies in somebody's mind relative to the other things they could choose. The relative part is what makes it hard. A claim that stands alone, such as being reliable or high quality, is not positioning, because it does not tell anybody what to pick you over. Positioning only exists where there is a comparison, stated or implied, and a reason to land on one side of it.
The most common error is treating positioning as a slogan and stopping there. A slogan is an output. Positioning is a decision that has to exclude things: an audience it is not for, a use it does not serve, a way of working it declines. The blunt test is whether a direct competitor could sign your positioning statement without amendment. If they could, you have written a category description rather than a position, and every piece of content built on it will read like everybody else's.
The second error is assuming the alternatives are competitors. Usually they are not. The realistic alternatives are doing nothing, carrying on with a spreadsheet, asking a friend, hiring somebody junior, or using a general-purpose tool for a specific job. Positioning against the substitute people actually use is far more productive than positioning against the company you find most annoying, and it changes which arguments you need to make.
A workable statement has four parts: the specific audience, the alternative it is being weighed against, the benefit that matters most to that audience, and the reason the claim is credible. Each part should be concrete enough that it could be wrong. Written that way, positioning becomes a filter rather than a plaque. Proposed content that does not serve the named audience, or that argues a benefit you never claimed, can be declined without a debate about taste.
Positioning is also the field that goes out of date first and is noticed last. The offer changes, the audience quietly moves upmarket, and the written position keeps describing last year's business while everything downstream is still generated from it. Reviewing it on a schedule, and treating a change to it as a change to everything derived from it, matters more than getting the first version perfect.