Effective Health/Effective Hit Points is the term we use for hit points plus relevant guaranteed damage reductions like Defense or MR--basically, it's the amount of damage that would cause the unit to keel over from one swing. It's handy to know but obviously isn't the only relevant calculation for tanking--e.g., bandits tend to have high effective health but they're skewed so heavily towards pure hit points that they often require an impractical amount of healing support to weather multiple hits. Leeanne's at the opposite extreme; her health is marginally lower than CC60 Garrett's and often far lower than that of a bandit but despite being a silver she's skewed so heavily towards pure defense that she can tank multiple fast attackers just fine when her skill is up.