Personal brand
The reputation a specific person holds in the minds of people who encounter their work: what they are known for, who they are useful to, and how they sound. It exists whether or not it is managed, because audiences form impressions from whatever evidence is available to them.
Personal brand is not Personal branding.
A personal brand is the asset, the impression that already exists in other people's heads whether you tend it or not. Personal branding is the practice of deliberately shaping that impression. You cannot choose not to have a personal brand; you can only choose whether to do any branding.
In practice
A personal brand is not a thing you build and then own. It is a reputation, and reputations live in other people's heads, which means the only part you directly control is the evidence you supply. Every post, talk, profile line and reply is a data point, and an audience assembles those points into a summary far cruder than the one you would write for yourself. Usually that summary runs to three or four words: the thing you are known for, roughly who you help, and whether you appear to be serious about it.
The common misreading is that a personal brand is an exercise in self-presentation, a matter of finding your authentic voice and becoming more visible. That framing produces a great deal of content and very little recall. The practical function is narrower and less romantic: it decides whether somebody who needs what you do thinks of you at the moment they need it, and whether the person they recommend you to can repeat your description without mangling it. Both are recall problems, and recall is bought with specificity and repetition rather than with range.
A second misreading is that you only have a personal brand once you have an audience. You have one the moment anybody forms an impression of your work, including a single client, a hiring manager or a former colleague. For most consultants, founders and specialists the brand already exists and is simply low-resolution, assembled from a job title, a headshot, an out-of-date bio and whatever was last posted. The question is never whether to have one, only whether the evidence you are putting into the world agrees with itself.
Used well, a personal brand is a constraint rather than an aspiration. Practitioners write the description down: who it is for, what it claims, what it will not talk about, and how it sounds. Then each piece of work is judged against that description instead of against how it feels on the day. The description outlives any individual post, because it is the only artefact that can be handed to a designer, reused by a writer, or checked when the work starts to look like somebody else's. Structured versions of this exist in software, where the description becomes a set of fields rather than a paragraph and the content is generated from those fields.
It should still change. Offers sharpen, audiences narrow, positioning moves. The difference between a brand that evolves and one that drifts is whether the description was updated first and the work followed, or the work wandered off and the description was never opened again.