Hello Tanya, "The" is a definite article... eg: The table here (we know we are talking about that table = definite "a" is an indefinite article... eg : a table (we don't know about that table; could be any table = indefinite).

Tanya, 1. a slow, stiff, haughty, or grim stride The indefinite article is used because "stalk" is defined here as one or several of the many different types of stride. It can be a slow, or a stiff, or a haughty, or a grim stride. 2. the act of stalking game, an enemy, etc. The definite article is used because "stalk" is defined here as ONLY the act (or process) of stalking, game, an enemy, etc ....not the stride, or any other aspect of stalking.

Marie has the best answer. You could say "He stalked his prey all night." and " He stalked across the room towards the door". The reason you use "the" in the definition is because of the sentence construction. "The act of _______-ing" "a" stride. VS "the act of" striding Hope that helps Matt