In cell culture
refers to the coverage or proliferation that the cells are allowed over or throughout the culture medium. For example a confluency of 40-60% will mean that there may be little or no restriction to the growth of the cells in/on the medium and they can be assumed to be in a growth phase.
This observation is often related to the color of the media supporting the cells (i.e. rate of consumption) and the number of dead, floating cells that have not necessarily attached to the plate.