Season
A season is one of the four yearly divisions of climate caused by Earth tilted axis, which changes the directness of sunlight and length of day.
Definition
Seasons are the four parts of the year, spring, summer, fall, and winter, and they happen because Earth's axis is tilted, so different parts of Earth lean toward the Sun at different times of the year. When a hemisphere is tilted toward the Sun, sunlight strikes it more directly and days are longer, so it receives more energy and has summer; six months later the same hemisphere is tilted away and has winter, the two hemispheres always have opposite seasons, and the equator has little seasonal change. In physical terms, seasons arise from the annual variation in solar insolation at each latitude, governed by the solar declination $\delta$, which swings between $\pm 23.5^\circ$ as Earth orbits with a fixed axis. Daily insolation depends on both the noon solar elevation, which sets energy per unit area through the cosine of the zenith angle, and the day length. Seasonal temperature lags the insolation peak by several weeks because oceans and land store heat, which is why the hottest weather comes in July and August rather than at the June solstice, and the eccentricity of Earth's orbit makes Southern Hemisphere summer, which occurs near perihelion, receive about $7\%$ more insolation than Northern summer.
Example
In July, North Carolina leans toward the Sun and has hot summer days, while Australia leans away and has winter. On the June solstice the noon Sun in Charlotte is $78.5^\circ$ high and daylight lasts $14.5$ hours; on the December solstice the noon Sun is only $31.5^\circ$ high and daylight lasts under $10$ hours, so less direct light for fewer hours means less heating. Sunlight arriving at a zenith angle $z$ delivers $S \cos z$ per unit horizontal area, where $S \approx 1{,}360\ \text{W/m}^2$ is the solar constant. For Charlotte at the June solstice noon, $z = 11.5^\circ$ gives $S \cos z \approx 1{,}330\ \text{W/m}^2$; at the December solstice noon, $z = 58.5^\circ$ gives about $710\ \text{W/m}^2$, nearly half.
Key Insight
Seasons are not caused by distance from the Sun; Earth is closest to the Sun in early January, during winter in North Carolina. Earth is at perihelion, its closest point, around January 3 and at aphelion, farthest, around July 4, and the $3\%$ difference in distance is far too small to cause seasons, working against Northern Hemisphere winter rather than for it. Milankovitch cycles, the slow changes in Earth's obliquity, eccentricity, and precession over tens of thousands of years, alter the strength of the seasons and are the pacemaker of ice ages. The same tilt that gives us summer and winter, varied slowly, has repeatedly buried North America under ice.