What is the difference between "ad valorum" and "ad valorem"? I am
reading a city charter document in the U.S. The sentence has to do
with tax on real estate property values and spells it "ad valorum" -
is this correct? Should it be "ad valorem"?

Hello cecropia,

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My research leads me to conclude that "ad valorum" is a misspelling of
"ad valorem."  The "e" spelling occurs much more often on the Web than
the "u" (on Google, approximately 719,000 versus 5,020).  If you
search for these terms in conjunction with the word "dictionary" or
"glossary," you'll see that only "ad valorem" appears in more
authoritative sources, such as:

"ad valorem tax"
Encyclopędia Britannica Online
http://www.britannica.com/eb/article-9003627

"Ad valorem tax"
Bankrate.com
http://www.bankrate.com/brm/definitions.asp?Page=1&channelId=10&slid=0&termUid=2296

If search for "ad valorem" and "ad valorum" together, you can see that
they're often used interchangeably:

"Glossary of Shipping, Insurance and Commercial Terms" [under "Ad Valorem (Lat.)"]
Sea Freight Council of Queensland
http://www.seafreightcouncils.com.au/glossary.htm

"Export-A" [under "Ad Valorem"]
EnviroTech Financial, Inc.

"Ad Valorem Tax Rate History" [with "u" form immediately above main heading"]
City of Greenville, Texas
http://www.ci.greenville.tx.us/lcd/city_government/finance_and_taxes/ad_valorum_tax_rate_history.asp

"Property Taxes" ["The Revenue Commissioner's office is responsible
for the assessment and collection of ad valorum taxes on your motor
vehicles.  ... Ad valorem taxes must be paid prior to the issuance of
the license plate."]
DeKalb County (AL) Revenue Commissioner's Office
http://www.dekalbrevenue.com/info.htm

See also:

"AD VALOREM - this service is withdrawn"
stuie's humble homepage
http://www.stuie.net/1pagers/adval.htm

I have seen no indication that these words are defined differently. 
My view is that "valorum" is a misspelling based on other other
Latin-derived words -- after all, we write "forum" and "decorum," not
"forem" and "decorem."

Just to be on the safe side, you might ask the city whose charter you
are reading -- the city's revenue department, perhaps -- about this
issue.  For all the dictionary definitions in the world, it's
conceivable that the opinion of city officials will be different.

- justaskscott

At the risk of Guillermo-ga's correcting me, 
"ad valorem" is the correct Latin expression:  -em, accusative case
following the preposition "ad".

"Ad valorum" is probably a now accepted mistake that started when
someone knew that it was Latin and assumed that -um was correct,  in
analogy to the nouns that Scott mentions.

 	
Ad Valorem

adj. Latin meaning "according to the value." This term is used in
reference to property taxes which are based upon the county's
assessment of the property's value. The value that is assessed is the
standard basis for local real property taxes, but some place maximum
amounts on the percentage of value or set "parcel taxes" that
establish a flat rate per parcel.

When I entered "ad valorum", my google.de immediately suggested that I
might want to search for "ad valorem".