Modular

Modular is a keyword ability that allows, for now only, artifact creatures to come into play with a number of +1/+1 counters, and redistribute the counters when the creature goes to a graveyard. Modular was released in 2004 the Darksteel set.

From the Comprehensive Rules:

• 502.35. Modular
  • 502.35a Modular represents both a static ability and a triggered ability. “Modular N” means “This permanent comes into play with N +1/+1 counters on it” and “When this permanent is put into a graveyard from play, you may put a +1/+1 counter on target artifact creature for each +1/+1 counter on this permanent.”
  • 502.35b If a creature has multiple instances of modular, each one works separately.