Full Idea
I |= φ means that the formula φ is true in the interpretation I.
Gist of Idea
I |= φ means that the formula φ is true in the interpretation I
Source
Wilfrid Hodges (First-Order Logic [2001], 1.5)
Book Reference
'Blackwell Guide to Philosophical Logic', ed/tr. Goble,Lou [Blackwell 2001], p.17
A Reaction
[There should be no space between the vertical and the two horizontals!] This contrasts with |-, which means 'is proved in'. That is a syntactic or proof-theoretic symbol, whereas |= is a semantic symbol (involving truth).