Abroad /əˈbrɔ:d/

adv. 到国外，在海外;广为流传地

It means that sb. goes to a foreign country, which is separated from the country where he/she lives by an ocean or a sea; or refers to something which is talked about or felt by many people.

它指的是某人去一个与自己居住国隔海相望的其他国家;或是指被很多人谈论或有所感受的事物。

They decided to spend their honeymoon abroad.

他们决定去国外度蜜月。

There was news abroad that a change was coming.

盛传即将有一个变动。

Aboard /əˈbɔ:d/

prep. 上车;在(船、飞机、车)上，上(船、飞机、车)

adv. 在船(或飞机、车)上，上船(或飞机、车)

It means on or onto a ship, plane, bus or train.

它指的是在(船、飞机、公共汽车、火车等)上;或是上了(船、飞机、公共汽车、火车等)。

Tom was already aboard the plane.

Tom已经登机了。

It had taken an hour to load all the people aboard the ship.

花了一个小时才让所有乘客都上了船。

最后总结一下，

这两个词虽然长得很相似，

但意思截然不同。

“abroad”是副词，

主要意思有两个，

“到国外，在海外;广为流传地”;

而“aboard”则为介词或副词，

意思是“在(船、飞机、公共汽车、火车等)上;

或是上了(船、飞机、公共汽车、火车等)”