There is no generally agreed definition of poverty. This is because, Piachaud argues, the definition of poverty is a moral question - it refers to hardship which is unacceptable.  'Poverty' may refer to:
The chart points to a range of different meanings: some are close to
each other, others far apart.
Poverty has often been described
in terms of low income and resources, but this does not capture the
huge range of issues which it touches. The
World Bank’s study, Voices of the
Poor, identifies several recurring themes. In relation to
material conditions, there are precarious livelihoods, problems of
physical health and living in excluded locations. The studies put
great emphasis on social relationships - relationships of gender,
social exclusion and lack of security. And then there are
political issues - limited communal organisations and abuse of
authority by those in power.  These issues cannot be set
aside in trying to deal with poverty, and poverty is
increasingly being thought of as a multidimensional concept.
Conventionally, poverty is represented in two main models.
Poverty, like all need, is defined in terms of the society where it takes place: what people can eat, and where they can live, depend on the society they live in. That does not mean that it is based only on a comparison with others in the same society; there are some countries where most people are poor. Beyond that, social and political relationships - like problems of gender or relationships to authority - are an integral part of the experience of poverty. These issues occur in many different societies, in poorer and richer countries alike, but they can only be understood in their social context.
The problems of poverty have been explained in many ways. Pathological explanations are those which attribute poverty to the characteristics or behaviour of poor people. They include:
Structural explanations explain poverty in terms of the society where it occurs. They include:
Because there is no agreed definition of poverty, there can be no agreed measure. Even if definitions were agreed, though, poverty would be complex and difficult to quantify. Measures of poverty have to be 'indicators', or signposts. The most commonly used measure is based on income. The World Bank, for example, used to use the arbitrary standard of $1.25 per day; at this level there are about 1300 million poor people in the world. At $2 a day, another arbitrary line, the figure approaches 2.5 billion.
Some nations apply 'budget standards', estimating the cost of a minimum basket of goods. The US defines its poverty threshold by identifying the cost of a food basket and estimating from that how much income is necessary. Others use relative measures. The European Union uses a comparative indicator which defines people as being 'at risk of poverty' if their income is below 60% of the median income. (The median comes half-way up the income distribution). This means that there is more poverty where there is more inequality, or 'economic distance'.
Social science surveys have estimated the numbers of poor people in various ways. Some use budget standards; others use a 'subjective' poverty test, to see whether people identify themselves as poor. Others again have developed a 'consensual' method, where an opinion poll is used to identify what people in that society see as essential, and working from there to see who can afford that standard. A survey for the Joseph Rowntree Foundation estimates that on these tests a quarter of Britain's population is poor. 
Providing services to poor people often depends on the idea that it is possible to 'target' resources or services by choosing the people who will receive them. In principle, people should receive benefits and services when they are poor, and they should not receive them when they are not. There are many problems with this in practice.
Because governments lack the capacity to test income effectively, some are using 'proxy' means tests, based on an analysis of national studies of households. Indicators are chosen which seem most likely to be associated with low income and poverty. These tests are easier and cheaper to run than full tests of income, and the information about whether people have basic assets is likely to be more stable. However, the World Bank has found that "it is hard to add new beneﬁciaries in the short term and hard to remove them from the program rosters when a crisis has passed."  The process is very approximate, which defeats the purpose of trying to fit the response to individual needs. It can seem arbitrary - some claimants tend to think it is decided almost as a matter of luck, some pray for success - and, Australian Aid has argued, there are openings for corruption, as people pay bribes to get on the rolls. 
The World Bank has also examined 'indicator targeting', trying to
reach the poor by focusing on associated factors. Concentrating on
areas affected by drought or crop failure; subsidies for
particular foods can be chosen in order to benefit poor people.
Sometimes universal services, like Essential
Health Care Packages, are better at getting resources to poor people
than selective services are.
Poverty can be responded to in many ways:
Any well-designed strategy to deal with a multidimensional set of
issues depends on a range of policies being implemented together, and
Poverty Reduction Strategies generally combine issues of economic and
human development with issues of governance. The most
effective responses to poverty have probably been economic
development, because it improves the conditions of many people
social protection, because it
reduces the circumstances in which people
are vulnerable; and political
empowerment, because it helps poor people
themselves to choose the issues which will make most difference to
their lives. Amartya Sen argues that democratic government is
critical to the position of the poor. 
The United Nations considers extreme poverty to be a breach of human rights. The Guiding Principles on Extreme Poverty and Human Rights begin with an understanding of poverty as “a multidimensional phenomenon that encompasses a lack of both income and the basic capabilities to live in dignity”.  States have duties, for example
The Millennium Development Goals, 2000-2015, had the aim of halving extreme poverty, measured at $1.25 a day; at that level, the World Bank estimates that 17% of the world's population are poor.  The Sustainable Development Goals for 2015-30 aim at the eradication of extreme poverty. The Guiding Principles, however, recognise that poverty is about much more than low income, and many of the issues that need to be addressed apply in richer countries as well as in poorer ones.
Spicker, The idea of poverty, Policy Press 2007