“indicator”是可数名词，意思是“指示器、指示牌、标志”。英英释义：something that can be regarded as a sign of something else. 我们来学习它作“标志”的用法，作这层意思讲 indicator 和 indication 可以同意替换。如果 A is an indicator/indication of B, 我们会习惯说“从 A 可以看出 B”或“A 是 B 的标志”。我们可以用 indicator 来替换 sign。
例句:
① 在一线城市拥有房产往往被视作是富有的标志，我们就可以说：
Owning a property in first-tier cities is widely regarded as an indicator of wealth/affluence.
② 国家之间外交关系的正常化往往是从元首的正式访问开始的，比如几年前奥巴马访问古巴，成为将近 90 年来第一位访问该国的美国领导人，就可以说：
Obama's visit to Cuba was an indicator of the improvement in relations between the USA and Cuba.
③ 从员工流失率可以看出一个公司的文化和竞争力，我们就可以说：
Turnover rates are a good/reliable indicator of a company's culture and competitiveness.
英文中有一个词专门表示“员工流失率”，那就是 turnover。
④ 最后来看几个《经济学人》中的例句：
1) Insects are indicators of ecosystem health.
2) When, in 2015, the government auctioned a site off the Massachusetts coast, the winning bid was $281,285—“a very clear indicator of a market that was basically dead”, says Thomas Brostrom, who leads the North American business for Orsted, a Danish energy firm.
3) Nelson Blackley of Nottingham Business School says that a strong indicator of the lack of confidence in the sector is that last year saw the lowest number of sales of malls since the recession in 2008.
1）翻译：
出国旅行已经不再被视为是富有的象征了。
Traveling abroad is no longer deemed an indicator of affluence. Traveling abroad has become an unreliable indicator of wealth.
2）造句
例子： The value of PM2.5 is frequently used as an indicator of air quality.
场景 ： PM2.5通常被用来作空气质量的指标
