a.

1. 无理性的
   Animals are irrational creatures.

   野兽是没有理性的动物。

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

   他有个荒谬的信念，以为人人都是他的仇敌。

3. 【数】无理的

adj.

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