slowing-down, deceleration
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ver-tra-ging, de ver-tra-ging-en]

"Vertraging" contains "traag", which translates to "slow".

a "vertraging" applies to a situation where something is slowing down
in general, the word is mostly associated with a delay in public

The term used in the physical sciences  – deceleration – also
translates to "vertraging". Its opposite – acceleration – translates to
"versnelling". The physical quantity inertia translates to

– "Mijn trein had vanochtend een vertraging van
20 minuten." 
("This morning my
train was delayed by 20 minutes." Lit.: "…had a delay of 20

– "Het project heeft een vertraging van
een half jaar opgelopen." 

("The project is delayed with half a year." Note the use of 'oplopen' in combination with 'vertraging': although the result is still the same, 'een vertraging oplopen' translates as 'to accrue a delay'. )

– "De tv-beelden van het WK in Zuid-Afrika worden met een kleine vertraging uitgezonden." 
("The TV pictures of the world championship football/soccer in South-Afrika are broadcast with a small delay." Note that 'WK' (from 'Wereldkampioenschap') is by default about football/soccer.)

– "Door persoonlijke omstandigheden is voor hem de studievertraging enorm toegenomen." 
("Due to personal circumstances the delay in finishing his studies has increased tremendously.")

Related words:

Vertragen: to delay, to slow down [verb] [vertraagde, vertraagd].
– Achterstand: arrears, backlog [noun] [de achterstand, de achterstanden].