An artifact creature is a permanent that has both the artifact type and the creature type at the same time. Under CR 301.1 and CR 302.1, a permanent's card types are not mutually exclusive — a single permanent can be an artifact, a creature, an enchantment, and more all at once.
When a card like Tezzeret the Seeker references artifacts (for his −X ability that fetches artifacts) or creatures (for other conditions), an artifact creature qualifies for whichever condition is being checked. The game looks at the permanent's current types as a whole snapshot, and both types are present simultaneously (CR 109.2).
This also matters for abilities that count or restrict by type. If an effect says 'target artifact or creature,' an artifact creature is a legal target via either clause. If an effect said 'for each artifact and each creature you control,' an artifact creature would be counted once for artifacts and once for creatures, contributing to both totals.
Concrete example: You control Ornithopter (an artifact creature). If you activate Tezzeret the Seeker's −X to search for an artifact with converted mana cost X or less, Ornithopter is a valid find because it is an artifact. It simultaneously also counts as a creature for any creature-based condition elsewhere in the same turn.
The key rule is CR 205.2a: a card can have multiple card types, and each type is fully operative. No rule causes one type to suppress or override another on the same permanent.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.