An acidic solution is an aqueous solution that has a pH less than 7. Aqueous solutions are liquid mixtures made up of any chemical dissolved in water. The reason why the solution must contain water is that pH is a measure of the number of ions formed in water.
One definition of an acid is a chemical that produces positive hydrogen ions when dissolved in water. That is why the pH scale is used as a measure of acidity, although it is in the literal sense only a measure of how many hydrogen ions are in a given aqueous solution. Stronger acids form more hydrogen ions. On the pH scale, they are given a lower rating. Neutral solutions on the scale are rated 7. Since pH is a logarithmic scale, acidic solutions separated by a single point on the scale are actually 10 times more or less acidic than one another.