>>280712610
It's a very broad definition
The specific criteria change with the times, but it really just means a girl who participates in traditionally masculine activities or interests, usually, with the reduction or, in extreme cases, exclusion of feminine activities or interests and often with an outspoken and boisterous personality as opposed to a demure one.
Historically, it just referred to girls who preferred to work or play outside instead of sitting in the parlor doing needlework.
As segregation of activities by gender continues to lessen, the standards by which one "acts like a boy" have become narrowed, requiring more extreme behavioral proclivities to still meet the classic definition in a modern context. Most often this manifests as a girl who is physically active, blunt, outspoken, keeps her hair short for practical reasons, is oblivious to romance, and has a lack of interest that borders on distaste for makeup and dresses. Some would say that a girl needs to have all of this to be considered a "tomboy" in the modern sense, but a classic tomboy only needed to exhibit two or three traits to qualify.
In general, it's an imprecise descriptor that encompasses a wide variety of more specific character archetypes such as Martial Arts/Sports Girl, Soldier Girl, Gamer Girl, or even Genki Girl or any other girl whose primary defining trait is traditionally associated with men or boys.
tl;dr
Tomboy is more genus than species
