"upgrade" versus "update".

[peter@easthope.ca]

I get that "dist-upgrade" is the replacement of a distribution 
by the subsequent distribution.  Replacement of wheezy by jessie 
for example.  What is meant by "upgrade"?  In many places it
appears to be synonymous with update.

For example, googling "site:debian.org update upgrade" yields, 
https://wiki.debian.org/UnattendedUpgrades 
"Running unattended-upgrades:
Its[sic] purpose is to keep the computer current with the latest security 
(and other) updates automatically."

Regards,                  ... Peter E.

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