1.Definition: existing in fancy only

"Fantastic figures with bulbous heads the circumference of a bushel"

Related Adjective(s):fantastical

2.Definition: extraordinarily good or great; used especially as intensifiers

"A fantastic trip to the Orient", "The film was fantastic!"

Related Adjective(s):grand, howling, marvellous, marvelous, rattling, terrific, tremendous, wonderful, wondrous

3.Definition: extravagantly fanciful in design, construction, appearance

"Gaudi's fantastic architecture"

4.Definition: fanciful and unrealistic; foolish

"A fantastic idea of his own importance"

Related Adjective(s):wild

5.Definition: ludicrously odd

"Fantastic Halloween costumes"

Related Adjective(s):antic, fantastical, grotesque