Vocabulary posters are a fantastic activity for pre-teaching vocabulary at the front of a new unit. Pre-teaching vocabulary is a big benefit for all your students not just your special populations. You can provide the definition or have students create/find their own definition. Have students use the word accurately in a sentence. Also have students demonstrate their comprehension of the word by representing it visually through a drawing or finding a an image.
Here is a Google Slides Template for vocabulary posters. You can assign this in Google Classroom or even just print it out and go old school with it. Be clear with your expectations for your students. Do you want them to copy your definition or create their own? Do you want the students to find an image or draw their own? I left the directions off so that you can customize that to your needs.