instant
1 [usually before noun] happening immediately 立即的；立刻的 immediate ： She took an instant dislike to me. 她立刻就讨厌我. ◊ This account gives you instant access to your money. 这个账户让你随时调动款项. ◊ The show was an instant success. 演出一炮打响.
2 [only before noun] (of food 食物) that can be made quickly and easily, usually by adding hot water 速食的；即食的；速溶的；方便的 ： instant coffee 速溶咖啡
instantaneous
adj. happening immediately 立即的；立刻的；瞬间的 ： an instantaneous response 即时的反应 ◊ Death was almost instantaneous. 当时生命垂危. in∙stant∙an∙eous∙ly adv.
作为英文的基本意思,instant的第一项解释同instantaneous的解释完全一致,意思也一样,但是,你看到1后面的括号了吗?它只能用在名词前.
而instantaneous位置灵活一些.