Impact is ...

... "adverse change to the level of business objectives achieved"
[source: 
ISO/IEC 27000]

... the inertial energy imparted by a moving mass impinging upon an object

... "the adverse outcome or consequences caused by or arising from an
information security incident, leading to direct and/or indirect
(consequential) losses/costs to the
organisations and/or
the individuals concerned" [source: SecAware glossary]

... the point when probability functions collapse

... when possibility becomes reality

... when threat meets vulnerability

... short, medium and long-term

... loss of control over an asset

... too late to prevent or avoid

... being smacked in the head

... when p(occurrence) hits 1

... when gloved fist hits chin

... what we tried to prevent

... what we sought to avoid

... an impressive entrance

... the resonance of a bell

... when risk eventuates

... when shit meets fan

... not too late to react

... being compromised

... a successful attack

... the point of failure

... adverse outcome

... the after-effects

... hard to quantify

... inconsequential

... career-limiting

... a wake-up call

... loss of control

... consequences

... being harmed

... consequential

... unanticipated

... ramifications

... a pivot point

... motivational

... the moment

... open-ended

... unexpected

... anticipated

... predictable

... memorable

... an incident

... a dull thud

... percussion

... disastrous

... dispersed

... an exploit

... negligible

... bad news

... predicted

... expected

... dramatic

... being hit

... a breach

... a failure

... a crater

... a driver

... focused

... harmful

... gradual

... striking

... serious

... a crash

... sudden

... general

... moving

... a miss

... severe

... shared

... crunch

... a dent

... costly

... trivial

... oh oh

... a flop

... hurty

... costs

... bang

... ouch

... a hit

...

Check out these elaborations for greater impact:

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