Stunting is easy to define--slow growth.
In panfish populations stunting usually occurs when there are so many panfish that there is not enough food to go around. That is why I crawl up on my soap box so often and state that largemouth bass are KEY in controlling numbers of panfish in pits and ponds.
However, what might be a density of panfish that causes stunting in one body of water may not be a density of panfish that causes stunting in another more productive body of water. For example, Nebraska's sandhill lakes are so productive, have so much habitat and associated food organisms, that you can have lots and lots of panfish and not have any stunting. On the other hand, a relatively new sandpit may be able to support far less pounds per acre of bluegills without stunting.
In fish stunting can also be compounded as slow-growing fish tend to become sexually mature at a younger age. Once fish become sexually mature, they devote more energy to the production of gametes and less to growth. That is why you want a population of bluegills where you have a bunch of those big parental males--those fish "put off" maturation in order to grow larger. Smaller males must delay maturation in order to reach a size where they can compete for females. In populations of stunted bluegills you likely will have more of those small, female-imitating, sneaky male bluegills that are sexually mature and will never grow much larger