This is a baseball term which refers to a pitcher whose style dictates they throw as fast and as hard as possible at all times. The most famous of all power pitchers is Nolan Ryan, who is in the Hall-of Fame for breaking and setting numerous pitching records, such as his MLB record 5,714 strikeouts. Although Nolan Ryan is famous for his fastball, a power pitcher must have many different types of powerful pitches in order to consistently strike out batters. Some examples include the slider, curveball, and change-up. This type of pitching is geared towards getting strikeouts, rather than relying on defense, to get outs.
There are fewer power pitchers in the modern game because it has been found that throwing in that style for extended periods of time can do damage to the throwing arm. Since modern day players have large contracts, teams are much more careful with pitchers who throw very hard, very often. Furthermore, players do not want to pitch hard because they know it can reduce their longevity and effectiveness. The pitch count became more popular when teams began awarding bigger contracts to power pitchers, and wanted to extend their usefulness to the team.