The phrase "percent decline" refers to the decrease or reduction of something in terms of a percentage. It tells you how much a particular quantity has dropped compared to its original value. Full definition
In 2013, for example, a 92 percent decline in new installations led to a temporary loss (PDF) of 30,000 jobs. (greenbiz.com)
Today, just over 100 nature - derived drugs are currently in clinical trials, a 30 percent decline from just a few years ago. (scientificamerican.com)
After 24 weeks of treatment, children in the parent - training group showed a 48 percent improvement on parent ratings of disruptive behavior compared to a 32 percent decline for parent education. (sciencedaily.com)