A solution is a homogeneous mixture. In a homogeneous mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving. Homogeneous means that the components of the mixture are uniform throughout.
Sea water is a homogeneous mixture. A homogeneous mixture is a uniform mixture. This means that a sample from one part will be identical to another. One sample of sea water from the sea will be the same as another from another part of the sea.