The International System of Units (SI) defines standard units for measuring length, mass, time, and temperature, which are the metre, kilogram, second, and Kelvin, respectively.

In the SI system, length is measured in metres (m). The definition of the metre is based on a fundamental physical constant: it is the distance that light travels in a vacuum in $1/299,792,458$ seconds. This definition guarantees that the metre remains a stable and unchanging unit, not reliant on any physical object.

Mass is measured in kilograms (kg). The kilogram was originally defined as the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder maintained at the International Bureau of Weights and Measures in France. However, efforts are underway to redefine the kilogram based on fundamental constants of nature, moving away from reliance on a physical artifact.

Time is quantified in seconds (s). The second is specifically defined as the duration of $9,192,631,770$ periods of the radiation emitted during the transition between two hyperfine levels of the ground state of the caesium-133 atom. This definition links the measurement of time to a reliable atomic standard.

Temperature is measured in Kelvin (K). The Kelvin scale is rooted in thermodynamic principles and is based on the temperature of an ideal gas at constant pressure. Notably, the Kelvin scale sets its zero point at absolute zero, the theoretical temperature at which all matter possesses zero thermal energy.

In summary, the SI units for length, mass, time, and temperature are the metre, kilogram, second, and Kelvin, respectively. These units are defined in relation to fundamental constants of nature, ensuring consistency and precision in scientific measurements.