directly contrasting or opposing something such that the things are not compatible with each other;
e.g. If someone holds political beliefs that are antithetical to yours, it can be difficult to be friends with them.
(adjective)
(literary) relating to or using the literary/rhetorical device of antithesis, in which parallel or opposing words or concepts are placed next to each other;
e.g. The author used an antithetical phrase when she said that it was a pleasant day outdoors but a gloomy one indoors.