A flow of seawater.
Ocean currents flow around the world, moving water.
They're formed by wind, density difference in water masses caused by temperature and salinity variations, and events such as earthquakes.
They effect weather and climates globally, a good example is Britain, if you look at where we are on the map, we are at the latitude of where at most countries, snow and ice cover much of it over winter, however, due to the warm current coming from the Gulf of Mexico, we stay warmer in those cold winter months, preventing much of the UK from looking like Canada which is straight across the pond from us, which as everyone knows, gets VERY cold in winter, but unfortunately, does not have the warm current it needs to keep it thawed out.
Hope this helps.