Final answer:
The solar constant is the term used for the rate at which solar energy reaches Earth's atmosphere per unit area per time, typically around 1400 W/m².
Step-by-step explanation:
The amount of solar energy reaching Earth's atmosphere per unit area per unit time, which is approximately 1400 W/m², is known as the solar constant. The solar constant is the rate at which energy from the sun, in the form of sunlight, is incident upon Earth's outer atmosphere.
This value is on average about 1,360 W/m² but usually rounded to 1400 W/m² for simplicity in many contexts. Although its value can slightly vary based on Earth's distance from the sun and solar activity, it's generally very stable and thus called a 'constant'.
The solar constant is an essential factor in understanding Earth's climate system and the calculation of solar energy available for solar power.