Splice

Splice onto [ quality] is a keyword ability that was introduced on certain arcane instants from the Kamigawa block that functions while the card is in your hand. It allows a player to essentially add the rules text of the card to another arcane spell they are casting. Since the card with splice remains in the player's hand, it can later be cast normally or spliced again onto another spell.

Splice onto Arcane
In the Kamigawa block, the splice mechanic was restricted to Arcane spells, a subtype of instants and sorceries ("splice onto Arcane"), but the rules leave the option for other variants of splice. Splice onto Arcane ended up being too parasitic (only allowing you to play with other cards from the same set) and never quite captured the feel R&D had been hoping for. "Splice onto Arcane" is now considered to be the classic example of a parasitic mechanic, though that is mostly caused by the "Arcane" part.

In design, splice onto Arcane originally worked from the graveyard. Mark Rosewater designed it as "Piggyback": a flashback-like cost that let you graft a kicker effect onto another card. In essence, it would be a portable kicker that lived in the graveyard. Once you cast it once, you could then add it onto other spells. The mechanic wasn't designed for a specific set, but was first considered for Fifth Dawn. Late in design for Champions of Kamigawa, Rosewater realized that it should have been "splice onto instant" but it was too late for development to be able to properly playtest it.

Splice onto instant or sorcery
For years, R&D talked about bringing back splice but on instants and/or sorceries. They talked about it for Return to Ravnica, but chose to only have new guild keywords, so it was off the table. That restriction no longer applied for Guilds of Ravnica, so they decided to give splice onto instants and sorceries another try for the Izzet League. In the end, it didn't live up to the hype they had been building up over the years, so they moved on to other options.

"Splice onto instant or sorcery" (As you cast an instant or sorcery spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card’s effects to that spell.) was finally introduced in Modern Horizons with Splicer's Skill and Everdream.

Reminder text
The original rules only allowed Splice onto [subtype]. Note that instant and sorcery are types, not subtypes. After introducing Splice onto instant or sorcery, the rules had to be changed accordingly.

The old reminder text was: Splice onto [subtype] [cost] (As you cast an [subtype] spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card's effects to that spell.)

The new reminder text is: Splice onto [quality] [cost] (You may reveal this card from your hand as you cast a [quality] spell. If you do, that spell gains the text of this card’s rules text and you pay [cost] as an additional cost to cast that spell.)

Rulings

 * You must reveal all of the cards you intend to splice at the same time. Each individual card can only be spliced once onto a spell.
 * If you have more than one card with the same name in your hand, you may splice both of them onto the spell.
 * A card with a splice ability can't be spliced onto itself because the spell is on the stack (and not in your hand) when you reveal the cards you want to splice onto it.
 * The target for a card that's spliced onto a spell may be the same as the target chosen for the original spell or for another spliced-on card. (A recent change to the targeting rules allows this, but most other cards are unaffected by the change.)
 * If you splice a targeted card onto an untargeted spell, the entire spell will not be resolved if the target isn't legal when the spell resolves.
 * If you splice an untargeted card onto a targeted spell, the entire spell will not be resolved if the target isn't legal when the spell resolves.
 * A spell does nothing on resolution only if *all* of its targets are illegal (or the spell is countered by an effect).