Mono, in the context of artifacts, is an obsolete term that used to denote an artifact as having an activated ability that included a tap symbol . The type was obsoleted with Revised. 
From the glossary of the Comprehensive Rules (Aether Revolt (January 20, 2017))
- Mono Artifact (Obsolete)
- An obsolete term that appeared on the type line of artifacts with activated abilities that caused the artifact to become tapped as a cost. Cards printed with this text have received errata in the Oracle card reference to simply say “Artifact,” and those abilities now include the tap symbol in their costs.