When two parties have a contract, but they tacitly rescind
it by letting it fall by the wayside, e.g. both sides over
time take actions which are not consistent with the agreement
at all, so a court can declare it invalid since both parties
were completely ignoring it. I just can't remember the word.
It's on the tip of my tongue. Thanks

Hello.

First of all, I must note that Google Answers provides general
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advice. If you need professional legal advice, you should contact a
qualified attorney in your area.

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The term is "abandonment."

"The general contract law test for abandonment of contracts has been
expressed as follows:
 A contract may be abandoned by mutual consent, and such consent may
be implied from the acts and conduct of the parties.   A contract will
be treated as abandoned when the acts of one party inconsistent with
its existence or acquiescence in by another.  Where acts and conduct
are relied upon to constitute abandonment, however, they must be
positive, unequivocal and inconsistent with the intent to be further
bound by the contract.  H.T.C. Corp. v. Olds, 486 P.2d 463
(Colo.Ct.App. 1971)."
source:
ABANDONMENT OF PRENUPTIAL AGREEMENT BY DONALD J. SASSER
http://www.aaml.org/Articles/2002-3/Abandonment-Sasser.htm

"Under the abandonment doctrine, once the parties cease to follow the
contract and the final project has become materially different from
the project contracted for, the entire contract is deemed abandoned."
source:
Pollak, Vida & Fisher: Government Liability Update
http://www.govlawweb.com/pubs/govlaw/govlaw_11.html?section=2

" Conduct of the parties that is positive, unequivocal, and
inconsistent with the existence of a contract is sufficient to support
a mutual abandonment of contract. Desnick v. Mast, 249 N.W.2d 878, 884
(Minn. 1980); Country Club Oil Co. v. Lee, 58 N.W.2d 247, 251 (Minn.
1953)."
source: Kimmons v. Innovative Software Designs, Inc., No. 00-6059 MN
(8th Cir. 09/20/2000)
http://bkinformation.com/NewCases/CaseCopies/in_re_innovative_software_design.htm

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Note that there's a related doctrine called "cardinal change."  See:
"Beyond Changes: Abandonment and Cardinal Change"  by Aaron P. Silberman
The Construction Lawyer, Fall 2002
http://www.rjop.com/publish42.htm 

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search strategy:
acts "inconsistent with" contract
"abandonment doctrine"
"theory of abandonment" etc

Thanks!

Thanks, that's what I was looking for.