The metre (meter in the US), symbol m, is the base unit of length in the International System of Units (SI). Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole (at sea level), its definition has been periodically refined to reflect growing knowledge of metrology. Since 1983, it has been defined as "the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299,792,458 of a second".

The first recorded proposal for a decimal-based unit of length was the universal measure unit proposed by the English philosopher John Wilkins in 1668. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the words metro cattolico (lit. "catholic [i.e. universal] metre"), which was derived from the Greek μέτρον καθολικόν (métron katholikón), "a universal measure". This word gave rise to the French mètre which in 1797 was introduced into the English language.

In 1668, Wilkins proposed using Christopher Wren's suggestion of a pendulum with a half-period of one second to measure a standard length that Christiaan Huygens had observed to be 38 Rhineland or 39¼ English inches (997 mm) in length. In the 18th century, there were two favoured approaches to the definition of the standard unit of length. One approach followed Wilkins in defining the metre as the length of a pendulum with a half-period of one second, a 'seconds pendulum'. The other approach suggested defining the metre as one ten-millionth of the length of the Earth's meridian along a quadrant, that is the distance from the equator to the North Pole. In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies slightly over the surface of the Earth, which affects the period of a pendulum.

Baby K (October 13, 1992 – April 5, 1995) was an anencephalic baby who became the center of a major U.S. court case and a debate among bioethicists.

Stephanie Keene, better known by the pseudonym Baby K, was born at Fairfax Hospital in Virginia, USA. At the time of her birth, she was missing most of her brain, including the cortex; only the brainstem had developed during pregnancy, the portion of the brain responsible for autonomic and regulatory functions, such as the control of respiration, the heartbeat and blood pressure.

The baby's mother had been notified of her condition following an ultrasound, and had been advised to terminate the pregnancy by her obstetrician and neonatologist but chose to carry the child to term because of "a firm Christian faith that all life should be protected". The hospital's viewpoint was that care provided to the baby would be futile. The mother's viewpoint was that mechanical breathing support must be provided during the baby's periodic respiratory crises. Fairfax Hospital doctors strongly advised a do not resuscitate order for the child, which the mother refused. Stephanie remained on ventilator support for 6 weeks while Fairfax searched for another hospital to which to transfer, but no other hospital was willing to accept her. After the baby was weaned off constant ventilator support, the mother agreed to move the child to a nursing facility, but the baby returned to the hospital many times for respiratory problems.