There are several uses for this word all are out of date during the time of Trek.
The First HackersEdit
The term hacker was first used to describe computer enthusiasts at the start of the Digital Age during the 20th century. It was a development of the kind of attitude that built radios sets or ground their own telescope mirrors before computers were common.
The First DefinitionEdit
Hacker was originally meant to describe someone who experimented with, was knowledgeable of, and could do things with computers. Programmers, engineers and people who did these things learning as they went, all were described as "Hackers" in he original computer enthusiast subculture. This sub-culture was active from roughly 1960 and 1995.
The Second DefinitionEdit
This is the more popularly known and used definition. It refers to a person who can use programming and technical acumen to gain unauthorized access to computers, usually secure systems like banks, military, government or other computers with high value content. Sometimes with criminal intent, sometimes "for the lutz".
Active during the "Second Hacker Age" in the 21st Century. Typically these criminals did damage engaging in spiteful and destructive behaviors. They generally making unpleasant messes for others to fix. They were also romanticized by fiction into rebels and anarchists resisting an oppressive order.
During the 21st century, hackers, console cowboys, slicers, and deckers became popular romantic figures, even though in reality they were largely thieves and con-men. Success in the field meant either making a "Big Score", that is, stealing a large fortune (so you could retire), writing a successful program or getting hired to do corporate programming and computer security.
In a few cases, hackers were at the center of adventurous and easily romanticized capers, and the resulting fiction romanticized the field even more.
In the modern day, many field engineers take inspiration from the romanticized image of the 20th and 21st century hackers. They share a similar fearless tendency to push technology way past its official limits.
It is not often mentioned, but much of Starfleet's software and equipment for talking to and understanding alien computers descends from software and techniques pioneered by the late hackers.