Mash
The mash is the brewing process that takes advantage of the enzymes present in the base malts in order to breakdown proteins and convert starches present in the grains. Certain temperatures and pH's are used in order to achieve these results by activating and deactivating specific enzymes, for example beta amylase and alpha amylase which are used to convert starches to fermentable sugars. The mash is where one can most influence the sweetness or dryness of the final beer as well as the alcohol present. You can do this by varying the temperatures used in your mash as well as the pH. For example if you would want a dryer less sweet tasting beer you might choose a longer rest during the beta amylase (140-149) range of activity with a short rest in the alpha amylase range (155-167) with the beta activity favoring a bit of a lower pH from 5.4-5.6 and the alpha favoring a bit of a higher pH from 5.6-5.8. One can also prolong the activity of these enzymes by employing a thicker mash but the mash will then need longer to convert since the enzymes are not as mobile since there is less liquid present and would need more time to be exposed to more of the grain. Whereas by using a thin mash the enzymes will denature when they are supposed to but you will need less time to convert all the starches.
or
The conversion that takes place in a mash. The result is wort.
The mashing process is done in a Mash Tun.
Resources:
1) Jean De Clerck, A Textbook In Brewing, Vol: 1 ;
3) http://www.mashers.org/mrlager.html
4) Siebel Institute Of Technology Lecture Material