Axis
An axis is the imaginary line through the center of a body around which it rotates; Earth's axis runs from the North Pole to the South Pole.
Definition
An axis is an imaginary line that goes straight through the middle of a spinning object; Earth's axis runs from the North Pole to the South Pole, and Earth spins around it once a day. The axis is tilted $23.5^\circ$ from a line perpendicular to Earth's orbital plane, and it keeps pointing in the same direction in space throughout the year, which is what causes the seasons. Physically, the rotation axis is the line of Earth's angular momentum vector, which stays fixed in inertial space by conservation of angular momentum unless a torque acts on it. The Sun and Moon exert a torque on Earth's equatorial bulge, causing the axis to precess in a cone of half-angle $23.5^\circ$ with a period of about $25{,}800$ years, plus a smaller nutation with an $18.6$-year period. Earth's geographic poles also wander by a few meters relative to the crust in the Chandler wobble, and the magnetic poles are separate from the rotation poles and move independently.
Example
A spinning top turns around its axis, and a basketball spinning on a player's finger rotates around an axis that runs from the finger up through the top of the ball. From Charlotte at $35^\circ$ N latitude, Earth's northern axis points to a spot in the sky $35^\circ$ above the northern horizon, marked closely by Polaris; from the equator Polaris would sit on the horizon, and from the North Pole it would be straight overhead. Because of precession, Polaris will drift away from the pole over the coming centuries, and in about $12{,}000$ years the bright star Vega will be the pole star. Around $2{,}700$ BCE the pole star was Thuban in Draco, which the Egyptian pyramid builders aligned to.
Key Insight
Earth's north axis points almost straight at Polaris, the North Star, which is why Polaris stays still while every other star seems to circle around it during the night. The axis does not wobble back and forth during the year; it points the same way in June and December, and what changes is which hemisphere is leaning toward the Sun as Earth moves around its orbit. Earth's axial tilt is unusually stable, varying only between about $22.1^\circ$ and $24.5^\circ$ over $41{,}000$-year cycles, because the Moon's gravity stabilizes it. Mars, with no large moon, has had its tilt swing by tens of degrees, with drastic climate consequences.