My kid (16 months now) is doing this developmentally appropriate, but incredibly annoying thing where she uses a word once and then it vanishes from her vocabulary. I'm assured that they will come back eventually, but it's a frustrating wait. Here is a collection of words she is using or has used:
"dada" an all purpose word that can mean daddy or anything she wants
"all done" sign language symbol
One time words:
"chow-ee bown" charlie brown
"teet" tweet -- what a bird says
"gae-doon" gate down -- what she says when I'm on the opposite side of a gate
"pree" not sure if pretty or green (pointing to the butterfly at the end of The Very Hungry Caterpillar"
"got it" when going for my mom's cats
There have been more, but those are the ones I can think of right now.