communication

n.

表达;交流;交际;传递;通信;交通联系;信息;书信;电话

例句：

The ambassador has brought with him a communication from the President.

communications

n.

表达;交流;交际;传递;通信;交通联系;信息;书信;电话

从单词a可以看出，后者是前者的复数形式。