A psi-ball is a type of construct. In fact, it is typically the most basic form of construct that newcomers to psionics are likely to encounter. It is simply a lump of "psi" energy, that the energy worker forms into a ball. The energy of the psi-ball is typically not very coherent. Psi-balls tend to dissipate after just a few moments without active concentration.
"Pinging" is the term used to describe the act of throwing a psi-ball at someone, typically another Sensitive. Often, the psi-ball is "charged" with an emotion or a sensation that the recipient attempts to perceive.