Defence skill in classic WoW is the stat that makes attacks fail against you; it is 0.04% each to be missed, dodge, parry and block per point of skill.
each point of defence above the attacker's weapon skill:
+0.04% chance to be missed, +0.04% dodge, +0.04% parry, +0.04% block
−0.04% chance to be critically hit
a mob's weapon skill = 5 × its level
Defence works against melee and ranged attacks only, through five small percentages at once. Gear with "+Defence" adds to the skill directly.
| +25 defence from gear | +1% each to be-missed, dodge, parry, block; −1% to be crit |
| 300 defence vs a level-63 boss (315 skill) | the boss gets the other side of the same table: 15 × 0.04% = 0.6% more hits and crits on you |
Last checked: unverified against Forever. These are the long-established values of the classic ruleset (the 1.x game of 2005-2006); World of Warcraft Forever is in beta and may differ. When you test this one in Forever, post your numbers in Theorycraft.