Compounding

“But it’s the Compounding that makes Miles so powerful. If your Allomancy and Feruchemy share a metal, you can access its power tenfold. It’s complicated. You store an attribute inside the metal, then burn it to release the power. It's called Compounding. By the legends, it’s the way the Sliver gained immortality.”

—Wax explaining to Marasi Compounding and Miles Dagouter's power.

Compounding is a technique for using Allomancy to enhance Feruchemy.

When a Metalborn has an Allomantic and Feruchemical ability in the same metal, they may store an attribute inside the metalmind, then Allomantically burn the power. This gives a supercharged burst of the Feruchemical attribute.

The Lord Ruler used this ability extensively, as he was both Mistborn and a full Feruchemist, so he could Compound in every metal. He stored and burned age in atium to effectively live forever, and also used gold to heal very rapidly, leading to rumors that he survived flaying and decapitation. Miles Dagouter used gold Compounding to heal almost infinitely, hence his nickname, Miles Hundredlives.

During the Final Empire, Steel Inquisitors did not Compound, though it was sometimes possible for them to figure it out. Compounding was the reason the Lord Ruler persecuted Feruchemists so extensively--he did not want Feruchemists to mix with Allomancers and create more Compounders.

While typically, Compounding refers to Allomancy enhancing Feruchemy, there is most likely a way for Feruchemy to enhance Allomantic abilities as well.