[more fantastic; most fantastic]
1 : extremely good
That meal was fantastic! [=excellent, terrific]
We had a fantastic [=great, wonderful] time!
2 : extremely high or great
The train runs at fantastic speeds.
3 also fantastical /fænˈtæstɪkəl/ : very strange, unusual, or unlikely
He concocted a fantastic [=wild, outlandish] scheme to rob the casino.