"Tack" is adhesiveness: how firmly a substance sticks or adheres to a substrate. The substance is normally an adhesive, but it can be an ink, for example. Tack is measured in terms of the joules of energy required to detach something (normally a piece of tape) stuck with the adhesive. The temperature (at 23ºC is added) because "tack" varies with temperature: that is, the temperature affects the adhesiveness of the adhesive.

"Tack is a key property that must be present in an adhesive in order to form a reliable bond. For substrates that are temperature sensitive, it is important that the adhesive raw materials exhibit significant tack below the critical temperature for that material where degradation can take place."
http://www.specialchem4adhesives.com/tc/polyurethane-dispers...

"Review of Methods for the Measurement of Tack"
[...] Within this standard [...] determination of loop tack is defined for both the open and closed side adhesive [...] The maximum force to remove the polyester tape is recorded."
http://www.adhesivestoolkit.com/Docu-Data/NPLDocuments/P A J...


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Note added at 18 hrs (2012-05-07 06:54:41 GMT)
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The word "tack" has many meanings in different contexts. A tack can be a small nail or pin. In sailing, it means the direction a boat sails in relation to the wind, or the corner of a sail. Hardtack or tack is a very hard unleavened ship's biscuit. Tack (or tackle) can also be the equipment you put on a horse to ride it. And these are just meanings of "tack" as a noun; as a verb it has others.

However, although we don't know the precise context here, none of the above seems to fit, since they do not explain the reference to joules, nor to temperature (I think 23 c must mean 23 degrees celsius). The only meaning that does seem to fit those details is the one I've mentioned: adhesiveness. It is used in this sense in various contexts:

Bookbinding:
"tack ( tackiness )
1. A general term applied to the state or property of tending to adhere, the relative stickiness of an adhesive film, the resistance offered by an adhesive film to division of the adhesive surface, or the resistance offered by the adhesive (while in the plastic state) to removal of the adherends. Some printing inks, varnishes, and similar viscous liquids also display tack. See also: BLOCKING (2 , 3 ). 2. That property of an adhesive which results in a bond of considerable strength immediately after application and contact under low pressure. 3. The resistance of an ink film to being split between two surfaces, such as between rollers, between plate and blanket, or between blanket and paper. (164 , 309 )"
Bookbinding and the Conservation of Books: A Dictionary of Descriptive Terminology, Matt T. Roberts and Don Etherington
http://cool.conservation-us.org/don/dt/dt3428.html

Note the reference here to "resistance" to division or removal; hence the measurement in joules.

Medicine:
"the degree of stickiness of an adhesive required to affix a therapeutic foreign substance such as a transdermal delivery device to the skin."
Mosby's Medical Dictionary, 8th edition
http://medical-dictionary.thefreedictionary.com/tack

Automotive engineering:
"Tack
- A term used to describe the sticky quality of a rubber compound.
- The sticky quality of an adhesive film, either while wet or after the film has set. Technically it is the pull resistance (measured in dynes) exerted by a material completely adhering to two surfaces being pulled apart."
http://www.motorera.com/dictionary/TA.HTM

Again, note here the reference to "pull resistance".

Paper making:
"Tack: Sticky property of paper and paperboard adhesive and glue-coating materials."
http://www.inpaper.com/glossary-t.htm