opponent、competitor、enemy、foe与rival这些名词均有“对手，敌手”之意。
opponent : 最常用词，通常指在争论、辩论、竞选或竞赛以及其它各种矛盾冲突中站在对立面的“对手”或“敌手”。
a political opponent
政敌
Leading opponents of the proposed cuts in defence spending will meet later today.
反对削减国防开支建议的带头人将在今天晚些时候会面。
competitor : 指为同一目标或目的竞争者，一般无感情色彩。
Their prices are better than any of their competitors.
他们的价格比所有竞争对手的都要优惠。
How many competitors took part in the race?
有多少选手参加了赛跑？
enemy : 多指充满敌意的仇敌。
He's made a few enemies in this company.
他在公司中和几个人结了怨。
Max stole Lee's girlfriend and they've been enemies ever since.
马克斯抢走了李的女朋友，从那以后他们就成了冤家对头。
political enemies
政敌
foe : 语气较强，多用于书面文字和诗歌中。指不仅敌对，而且很危险，怀有很深的敌意。
那个刺客是你的朋友还是仇人？
Is the assassin your friend or foe?
你应该与他通力合作，以铲除你们共同的仇敌。
You should work in harness with him to eliminate your common foe.
（拓展学习：小词详解 | foe）
rival : 指与某人目标一致而想赶上或超过他的人，匹敌者。
He beat his closest/nearest rival by 20 marks.
他以领先20分的成绩打败了和他水平最接近的对手。
The companies produce rival versions of the toy.
这些公司生产的玩具跟我们的产品争夺市场。