IPA: /rəˈpɪdəti/
KK: /rəˈpɪdəti/
The quality of being fast or quick; how fast something happens or moves.
The rapidity of the train's movement surprised everyone on the platform.
Rapidity → It is formed from "rapidus" (from Latin, meaning swift or quick) and "-ity" (a suffix used to form nouns indicating a state or condition). The word "rapidity" refers to the state of being swift or quick.
Think of 'rapid' meaning fast, and remember that 'rapidity' describes the state of being fast.