From an New York Sun article by David Twersky:
Javier Solana argued that Israel's actions could be seen as "not proportionate." Under international law, "proportionality" has nothing whatsoever to do with whether one party's response is proportional in some complex measurable way to the provocation of the other. Thus, Israel's robust reaction to the ambush and murder of eight soldiers and the kidnapping of two should not be measured by anything other than the standard as laid down under international law.
The relevant language in the Additional Protocol to the Geneva Conventions of August 1949, and Relating to the Victims of International Armed Conflicts Part IV, Article 51, part b states refers to "an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated."
In other words, proportionality is a concept that relates the direct military advantage anticipated (not even achieved but merely anticipated) and the pain and injury inflicted on civilian life and property.
Ed Lasky 7 21 06