a.

1. 無理性的
   Animals are irrational creatures.

   野獸是沒有理性的動物。

2. 不合理的；不明事理的；荒謬的
   He had an irrational belief that everybody was his enemy.

   他有個荒謬的信念，以為人人都是他的仇敵。

3. 【數】無理的

adj.

1. not logical or reasonable.
2. Mathematics  (of a number or quantity) not expressible as a ratio of two integers (e.g. π).