What counts as gentrification?

In response to what I wrote about gentrification not driving low-income minorities from their neighborhoods, the reader tegwar raises a very good point: What exactly do we mean by “gentrification?” He writes:

I’m a touch underwhelmed on first glance. They define gentrifying neighborhoods as those which were in the bottom quintile by

  1. 1
  2. ...
  3. 900
  4. 901
  5. 902
  6. ...
  7. 1014