The Glock-18 is the terrorists' standard starting pistol in most versions of Counter-Strike.
At the beginning of a normal round, the terrorist side generally starts with the Glock-18, while the counter-terrorist side receives a different default pistol. The Glock-18 is notable for its selectable firing modes, allowing semi-automatic or burst fire in Counter-Strike versions that support the feature.
The exact pistol loadout can vary by game version or special mode, so “most versions” is important. In Counter-Strike: Global Offensive and Counter-Strike 2, the terrorist default pistol is the Glock-18.
The Glock-18 is often compared with the USP-S, but the USP-S is a counter-terrorist sidearm. Players can purchase or use other pistols later, but the starting weapon is determined by team and game rules.