"Casual language" refers to a relaxed and informal way of speaking or writing, using everyday expressions and slang, instead of formal or professional language. Full definition
By using casual language, you immediately put your learner at ease. (elearningmind.com)
Sometimes, we use formal assessment language in our tests and assignments, but use more casual language in class. (edutopia.org)
Make sure that your emails stand out because of the content, and not because of sloppy mistakes, poor formatting, or overly casual language. (thebalance.com)