The text of Dungeon World mentions “hits” and “misses”, but doesn't seem to define these anywhere. A miss is obviously a 6− result, but what is a hit? Is it a 10+ or a 7+?
Yes, this is an oversight in the text of Dungeon World. The game mentions hits and misses, even using them in rules, but they aren't defined anywhere. In the obvious place to look for the definition, Moves and Dice in the Playing the Game chapter, only the three outcomes are defined.
Fortunately we can divine the definition without much trouble, and it is…
A 7+ is a hit
A hit is anything that isn't a miss — that is, a roll of 7+.
We can see this from three things:
The development history of the game.
Dungeon World was originally a “hack” of (that is, a supplement for) Apocalypse World and required that game's book in order to have a complete set of rules and be able to play.
In Apocalypse World, a hit is defined as a 7+ (AW 1e, p. 13; AW 2e, p. 10; emphasis mine):
If the sum total is 6 or less, that’s a miss. If it’s 7 or more, it’s a hit. 7–9 is a weak hit, 10+ is a strong hit.
Structurally the way moves are rolled for in Dungeon World is unchanged from Apocalypse World, so there's no structural gap that would allow the definition to have been changed — especially not without saying so!
How the three outcomes are characterised is consistent with AW.
The only time the three outcomes are collected together into two categories are their initial definition and in the section on Custom Moves in the GM's rules. Anything 7+ is a “success” — a 10+ being a complete success and a 7–9 being a success, albeit partial. “Miss” is referenced in many moves (e.g., Spout Lore, Last Breath), though never defined, as what happens when a success doesn't happen.
And because they're consistent with the definitions of hit and miss in Apocalypse World, we have support for concluding that's the omitted definition.
Dividing the outcomes into 6− misses and 7+ hits are natural categories already present in DW.
These outcomes form two natural categories. The miss category is already defined implicitly, by the way moves reference it, as 6−. The other natural category present in DW's text is the “success”. These two concepts are complementary and binary. Assuming “hits” exist in DW (and they do, because the game references them in its rules), then it must be the complement of a miss. And since the other two outcomes are explicitly called “success”, that's our natural category that matches a hit, both because it's a synonym and because the logic of these binary natural categories demands it.
The strongest evidence is that Dungeon World is, historically, an add-on to Apocalypse World's rules, and nowhere does Dungeon World explicitly rewrite what it means to get a hit. It just lacks explicit confirmation that it isn't changing the definition when it uses it, which it ideally would have done when it was rewritten to be a stand-alone game. Structurally, the way move rolls are made and the way moves reference “hit” and “miss” hasn't changed since its “hack” days, supporting that there is no change in their definitions either.
The way the Dungeon World text parallels Apocalypse World's definition, and the way that the three outcomes form natural categories that match the original game's definitions of hit and miss, just supports this conclusion.