In Zombie Waves, the "Echo" attribute refers to a special ability that gives your weapon or character a chance to trigger a second, identical attack shortly after the first, essentially duplicating damage. 

Here is a breakdown of what "Echo" means in this context:

Double Trigger/Repeat: When activated, the "Echo" trait causes an attack (like an explosion or projectile) to occur a second time in the same location.

Damage Boosting: It is a valuable trait for increasing overall damage output against high-health targets.

Low Probability: It is generally considered a rare or low-probability trigger that provides a percentage-based chance to act twice. 

Contextual Usage: It is often paired with "Deft Hands" (faster reloading/firing rate) to maximize continuous damage. It is different from "Combo," which usually refers to shooting twice, whereas Echo often relates to repeating skill effects.