Translation:The students are going to define the words.
I think both can be used, but to different effect. definer palabras to me is a general description, without any reference to any specific set of words. For example, "as part of the creative writing course, the students will define alien words to be used in their stories."
definer las palabras means to define a specific set of words, without actually having to list them. Example; "Tomorrow the students will define the words we have learned this week; then they will use them in sentences."