Tide
Tides are the regular rise and fall of ocean levels caused mainly by the Moon gravitational pull on Earth, with two high and two low tides most days.
Definition
Tides are the regular rise and fall of ocean levels along the shore, caused mainly by the Moon's gravity pulling on Earth's water and, to a lesser degree, by the Sun's; most coasts see two high tides and two low tides roughly every day. The Moon pulls harder on Earth's near side than its far side, stretching the oceans into two bulges, one facing the Moon and one opposite it, and as Earth rotates through both every $24$ hours and $50$ minutes, because the Moon has moved along its orbit meanwhile, most places cycle through two highs and two lows on that schedule. The underlying tidal force is the gradient of gravitational pull across Earth's width, scaling as $2GMR/d^3$ for a body of mass $M$ at distance $d$ acting across radius $R$, and that $1/d^3$ dependence is why the Moon's tidal effect works out to about $2.2$ times the Sun's despite the Sun's far greater mass. The resulting equilibrium tide would be under a meter, but real coastlines amplify or dampen it through basin shape, resonance, and Coriolis-driven rotation around amphidromic points, while tidal friction slowly lengthens Earth's day and pushes the Moon outward.
Example
At a North Carolina beach the water may reach far up the sand in the morning and pull back to expose tide pools by afternoon, with about $6$ hours separating high tide from low. Because the Moon moves ahead along its orbit, each day's high tides arrive about $50$ minutes later than the day before, and the Sun's tidal pull, despite the Sun's enormous mass, comes out a little less than half the Moon's because the Sun sits so much farther away. The Bay of Fundy in Canada sees tidal ranges up to about $16\ \text{m}$ because its length matches the natural resonance period of the tidal forcing, while the North Carolina coast typically ranges only about $1$ to $2\ \text{m}$, and parts of the Gulf of Mexico get just one tide a day.
Key Insight
There are two high tides a day rather than one because the Moon pulls the near-side ocean into a bulge while pulling Earth itself away from the water on the far side, raising a second bulge there. The Moon's gravity at Earth is actually weaker than the Sun's, yet the Moon dominates tides, because tides come from the difference in pull across Earth's width, and that difference falls off with distance much faster than the pull itself does. Tidal forces act on solid ground too: Earth's crust rises and falls about $30\ \text{cm}$ twice a day, Jupiter's moon Io gets kneaded by Jupiter's tides into the most volcanic body in the solar system, and near a black hole tidal forces would stretch a falling object lengthwise, a process nicknamed spaghettification.