The G2 refill is an ISO 12752 standard format refill that is very commonly used in retractable ballpoint pens with a click mechanism. This refill is 9.84 cm or 3.875 inches long and approximately 5.8 mm or 0.22 inches in diameter and comes in gel ink or ballpoint (oil-based) ink versions.
G2 format refills are believed to have originated in the iconic Parker Jotter and are used in many brands of pens including Aldo Domani, ACME, Aurora, Bexley, Brossert & Erhard, Colibri, Delta, Diplomat, Elysee, Faber Castell, Foray Focus, Hauser, Inoxcrom, Itoya, Jean Pierre Lepine, Krone, Marlen, Montegrappa, Monteverde, OMAS, Parker, Pelikan, Retro 51, Rotring, Schmidt, and Schneider.
If you have a pen with a retractable click mechanism then it is likely that it uses this kind of refill.
