From the SRD, under Epic magic items:
- Grants a bonus on attacks or damage greater than +5.
- Grants an enhancement bonus to armor higher than +5.
- Has a special ability with a market price modifier greater than +5.
- Grants an armor bonus of greater than +10 (not including magic armor’s enhancement bonus).
- Grants a natural armor, deflection, or resistance bonus greater than +5.
- Grants an enhancement bonus to an ability score greater than +6.
- Grants an enhancement bonus on a skill check greater than +30.
- Mimics a spell of an effective level higher than 9th.
- Has a caster level above 20th.
- Has a market price above 200,000 gp, not including material costs for armor or weapons, material component- or experience point-based costs, or additional value for intelligent items.