In the UK ebook versions, the first explicit mention of "apparating" or "apparition" appears to be in Prisoner of Azkaban:
All around them, people were asking each other the same question: ‘How did he get in?’
‘Maybe he knows how to Apparate,’ said a Ravenclaw a few feet away. ‘Just appear out of thin air, you know.’
POA: CHAPTER NINE – Grim Defeat
