'Northwood' means the northern wood – the wooded land north of Ruislip.
The name was first recorded in 1435 as Northwode, from Old English north and wode, and refers to the wood's position in relation to Ruislip and Ruislip Woods. The Domesday Book of 1086 credits the parish of Ruislip with immense woodland, enough for 1,500 pigs. The hamlet of Northwood grew up along the Rickmansworth–Pinner road.