Definition of height in Tree (set theory). Maybe give both definitions and say they're equivalent.
Add section for Σ21 definition of Continuum hypothesis. Point out that it's simpler than the definition given at the top of the argument (which call "weak CH"), equivalent given AC, but AD implies weak CH but refutes CH. CH relativizes up between models having same reals.
Create Structure (mathematical logic) page? Some overlap with model theory, but the latter should really discuss the things that are actually called model theory (stability, saturation, homogeneity, etc) rather than Tarskian structures and semantics.