It is widely agreed that eliminating extreme poverty in the world should take priority in thinking about our development goals going forward. The '$1 a day' poverty line is a simple metric for monitoring progress toward that goal. It was chosen in 1990 as a typical line for low-income countries (as explained in Dollar a day revisited). By this measure, poverty in the world as a whole is judged by a common standard anchored to the national lines found in the poorest countries. On updated data, the current value of this international line is $1.25 a day at 2005 purchasing-power parity. Today about 1.2 billion people in the world live in households with consumption per person below this frugal line. Thankfully, the world has made progress in bringing this count down; 1.9 billion people lived below $1.25 a day in 1990.
Notice that I say 'consumption' not income. A standard measure of household consumption is preferable as a measure of current economic welfare than income, and is typically measured more accurately than income. Fortunately, two-thirds of developing countries now have consumption-based poverty measures, although some regions, such as Latin America, are lagging in this respect.