Ping refers to a residual ringing sound heard after a keyswitch with metal springs inside has been struck.
The concept of ping has been a hotly debated topic, because the exact level of noise expected to be produced from metal switches is largely subjective. The perception of the problem is exacerbated by Cherry MX–based keyboards exhibiting non-uniform ping across the board, suggesting that some switches in a keyboard are "bad" in some way.
There have been widespread complaints about excessive ringing from certain keyboards, most particularly the Filco Majestouch 2 after it was first released, countered by others as being completely expected and normal behaviour for mechanical switching. Various solutions have been suggested, some of which implying that the level of ping in individual Cherry MX switches is partially down to the way that the main spring is assembled within each particular switch.
The only keyboard where the manufacturer has confirmed that the level of residual ringing produced by the switches is abnormal is Matias: intense ringing with the Tactile Pro 3 was found to be a design flaw with the Alps SKBM White switch, where the overly thin leaf springs (compared to Alps SKCL/SKCM series switches and clones) are prone to vibrating in unison when a key is struck. The new Matias switch has been designed with this in mind.
Buckling spring switches
One type of switch where ping is most definitely to be expected is the buckling spring switch; in particular the original capacitive version found in the IBM Model F is known for exceptionally loud ringing noise.