A baby hears the word Daddy in the presence of one big hairy guy that grins and cuddles and threatens to eat her toes and the word Daddy gets connected neurologically to the feel of a beard, the sound of a deep male voice, a silly grin and the sense of being loved. Every experience that this child has with her father becomes part of the meaning of Daddy. Gradually the word Daddy is understood as more than a label for a beloved man. Daddy is the word this child can utter to call her dad when he is in a different room, to complain to him when he says it is bedtime, and eventually to bring him to mind sixty years later when her father has been gone from this earth for a decade.
We learn words within social experiences. I was in The Netherlands in August for long enough that I began to understand some Dutch words. The first time a store clerk asked me in Dutch if I needed a shopping bag, I did not understand and she switched to English and repeated herself. By the time I left, three weeks later, I did understand and would shake my head no and pull out the bag I carried with me. I learned the words in that particular social context but now, here, I have no idea what the word for shopping bag is in Dutch. If someone said the word to me here, I don't think I would understand it.
Children with autism often have unusual difficulty learning vocabulary but like all learners, they need to learn vocabulary in a variety of contexts in order to understand the varied and nuanced meaning of words. In fact, it appears they learn new vocabulary like I learned the word for shopping bag in the Netherlands. They tend to learn the word in one social situation and not even recognize it as the same word in a different social situation. If we want the child to understand the word in a variety of social situations, we need to teach the word in a variety of social situations.
Example of different social contexts for the words shopping bag:
Let's go buy a new toy and put it in this shopping bag.
Help mommy take cans out of my shopping bag.
I can't find my shopping bag--help me look for my lost shopping bag.
The shopping bag is too full, we can't carry it.
You carry the shopping bag, my hands are full.
I have a surprise for you in the shopping bag!
Now if someone in The Netherlands had used the Dutch word for shopping bag in all of these contexts, I would probably still know how to say it today.