warning          
noun [ C or U ] uk/ˈwɔː.nɪŋ/ us/ˈwɔːr.nɪŋ/
B1 警告;告诫;提醒
something that makes you understand there is a possible danger or problem, especially one in the future
  Completely without warning, he turned up at my door with all four children! 事先都没说一声,他就带着全部4个孩子出现在我的门口! 
 There's a warning on the cigarette packet that says "Cigarettes cause cancer". 香烟的包装盒上有一条警告:“烟草严重危害健康。” 
 I'm not surprised you feel sick after eating so much - let it be a warning to you! 你吃了这么多感觉不舒服,我一点也不意外——就权当是对你的一个警告吧! 
 Just a word of warning - restaurants in this area can be very expensive. 只是提醒一句——这一带的饭馆都特别贵。 
formal The Health Minister issued a warning about the dangers of sunbathing. 卫生部长就日光浴的潜在危险发布了一条警告。 
 They can't dismiss you just like that - they have to give you a written warning first. 他们不能那样就把你解雇了——他们必须事先给你一份书面通知。 
 The police fired warning shots but the rioting continued. 警方鸣枪示警,但示威者根本不予理会。 
 warning sign (预示体内有某种疾病的)警示症状,预兆
a physical condition that shows someone has a disease
  The warning signs of the illness are respiratory problems and dizziness. 预示可能患有这种疾病的症状是呼吸困难和头晕。 
 - More examples
 - Police have put out a warning to people living in the area.
 - The judge said that the fine would serve as a warning to other motorists who drove without due care.
 - All cigarette packets carry a government health warning.
 - It would be irresponsible to ignore these warnings.
 - The police are trying to sift out the genuine warnings from all the hoax calls they have received.
 
Idiom   
hear warning bellsidiom (also warning bells start to ring/sound) 察觉不祥之兆;警钟已经敲响
used to describe a situation when people see signs that something bad has started or is going to happen
  For me, the warning bells started to ring when she lost all that weight. 她体重严重下降时,我就觉察到不对劲了。