How to avoid doing it: 

1. Get the students to read the exercise completely before starting to actually do anything. They can underline the words they don't know, or (more positively) underline the words they do know. 2. When a student asks you to explain the meaning of a word, DON'T. Ask the other students if anyone can explain it.

2. You could put the students in pairs or small groups and get them to explain the words they don't know to each other.

3. It's a good idea to get the students to try to guess the meaning of the word from the context it's in.

4. Get the students to look the word up in a (preferably English to English) dictionary.