Web, domains and email
FID
Abbreviation for First Input Delay
In short and in plain language
FID was a measure for web pages. It showed how long a page took to react to the first input of a visitor.
It has now been replaced by a newer measure. That one looks at the input during the whole visit.
Definition
FID was a metric for the delay before a page could respond to a visitor's first input. It has since been replaced by INP, which takes the interactions of an entire visit into account.