We may already be past the definition stage, but if not:
Portobello are overgrown crimini. Properly speaking, there's no such thing as a baby portobello or any of the other bello diminuitives, only superannuated criminis. Because of the commercial success of portobellos, produce sellers decided to rename criminis. As iplay pointed out, they are the same species (agaricus bisporus) along with buttons.
When the mushrooms are young, white and naive, they're typically called "button." After they age and dry out a little, they darken and the gills start to open. Then they're "crimini" -- the identification is especially easy if they wear they wear a gold chain and an open shirt. As they dry and open further (usually growing quite a bit larger), and get some expensive loafers, they become "portobello."
Fuhgeddaboudid,
BDL