Standard of livinginflation adjusted) income per person, although sometimes other measures may be used; examples are access to certain goods (such as number of refrigerators per 1000 people), or measures of health such as life expectancy.
The idea of a 'standard' may be contrasted with the quality of life, which takes into account not only the material standard of living, but also other more subjective factors that contribute to human life, such as leisure, safety, cultural resources, social life, mental health etc. More complex means of measuring well-being must be employed to make such judgements, and these are very often political, thus controversial.
However, there can be problems even with just using numerical averages to compare material standards of living, as opposed to, for instance, a Pareto index. Standards of living are perhaps inherently subjective. As an example, countries with a very small, very rich upper class and a very large, very poor lower class may have a high mean level of income, even though the majority of people have a low "standard of living". This mirrors the problem of poverty measurement, which also tends towards the relative.