Metamagic
Metamagic is a game mechanic created in Dungeons & Dragons 3rd Edition. As the name implies, it is a way for wizards and sorcerers to get even more powerful by tinkering with certain aspects of their spells, augmenting them in various ways, such as removing the need for certain components, expanding their area of effect, and so forth.
In 3rd edition, metamagic was handled as a family of feats that casters could pick up. Each metamagic feat represented a specific magical trick that a caster could apply when preparing or casting a spell, depending on their casting style, and usually with some drawback - mostly in the form of requiring a higher spell-slot than before. Multiple metamagic effects could be stacked on the same spell, but this could get prohibitively expensive in terms of spell slots.
Some metamagic feats could be taken multiple times. Most prominently, the Energy Substitution metamagic feat, which was broken up into five subfeats; Acid, Cold, Electric, Fire and Sonic. Each version of Energy Substitution taken allowed the caster to replace the normal energy type of their spell with that feat's type; Energy Substitution (Acid) could be used to create a Chain Lightning spell that did acid damage, whilst Energy Substitution (Cold) could be used to create Fireball spell that did cold damage.
Metamagic vanished in 4th edition, because it just didn't quite work with the new paradigm to how spells worked. Its spirit lived on in the form of various feats that provided permanent innate augmentations to different spells in a broad sense.
It returned in 5th edition, now relegated to a class feature for the Sorcerer, which could burn "sorcery points", a pseudo-mana system, when casting a spell in order to activate metamagic effects; sorcerers can now choose which metamagics they learn as they level up. Unfortunately, due to various issues...namely, how few sorcery points they get, the fact sorcery points are also used to replenish depleted spell slots, and the fact that sorcery points only recharge on a long rest... well, it makes metamagic a lot less useful in 5e than it was, which has upset many sorcerer fans.
It's telling that the Loremaster Arcane Tradition for wizards in Unearthed Arcana didn't once mention the word "metamagic", and yet it still manages to blow sorcerers out of the water in terms of being able to tweak and modify spells on the fly for greater efficiency in battle.