In Jewish terms, the party is secondary.
A bar mitzvah marks religious adulthood
Britannica explains that a bar mitzvah marks the coming of age of a Jewish boy at 13 and his acceptance of responsibility for the commandments.
That is the center of the event. A boy is no longer treated simply as a child in matters of Jewish obligation.
The ceremony grew over time
Britannica notes that while the age of religious majority is ancient, the fuller ceremony and celebration developed later.
This matters because bar mitzvah is both law and custom. The status matters more than any single ceremonial format.
Torah reading made the milestone public
Britannica describes the aliyah, Torah reading, Haftarah, sermon, and related honors that often shape a modern bar mitzvah service.
These practices give public expression to a new level of communal responsibility.
Why it still matters
Bar mitzvah still matters because Judaism treats adulthood not only as a personal feeling but as entry into commandment, literacy, and public responsibility.
The shortest accurate answer
A bar mitzvah is the Jewish coming-of-age milestone at which a boy, traditionally at 13, assumes religious adulthood and responsibility for the commandments.