Personal development is the cultural language of making a better self.
Modern life increasingly presents the individual as an unfinished project. A person is expected to identify deficits, set targets, acquire techniques, and demonstrate measurable progress. That pressure can produce useful learning, but it also makes the self the default site of explanation for problems that may be social, economic, spiritual, or institutional.
Personal development is the broad category for intentional efforts to expand a person’s capabilities, understanding, habits, relationships, or quality of life. It includes education, mentoring, coaching, reflection, goal setting, and other practices that aim at change across a lifetime. It is wider than the commercial self-help market and does not require a promise of constant performance or a purchasable solution.
The category becomes confused when development is treated as an end in itself. Improvement needs a standard: better by whose measure, toward what purpose, and accountable to which conception of a good life. Without that standard, personal development can collapse into self-optimisation, where every part of life is converted into a target, or into consumerism, where development is represented by what a person buys.
Source: Encyclopedic overview of personal development and self-improvement, https://en.wikipedia.org/wiki/Personal_development. This source is descriptive rather than peer-reviewed.