#1. A tool used, often in the form of an emoticon or acronym, that is relied upon to make a point or convey an emotion that the writer or speaker cannot do with their limited vocabulary.
#2: The use of acronyms like BRB, ROFL, OMG and WTF in spoken conversation as opposed to actually laughing, showing disbelief etc.
#3: A word, phrase or acronym that is so ingrained in someone's head, that they use without thinking if it's appropriate or even makes any sense at all.
Example of #1: She uses xD as a communication crutch because she isn't actually funny enough to make me laugh with only her words. She has to add that otherwise we wouldn't know it was a joke.
Example of #2: A person who uses 'ROFL' when speaking to them instead of actually laughing. This is a communication crutch because they either don't get the joke, don't know how to laugh or are just condescending assholes.
Example of #3: "I'm pretty sad, my grandma just died"
"That sucks lol"
"Why are you laughing about my grandma dying?"
"Sorry about that lol"
No matter what you say, this person has the communication crutch 'lol' and will automatically type it in every response.