[[$\text{pH}$]] = a measurement scale indicating the amount of hydrogen ions [[$\text{H}^+$]] in a unit of solution. As an equation,
[[$$\text{pH} = -\text{log}\{\text{H}^+\}$$]]
and as s a simplified equation,
[[$$10^{\text{pH}} \approx \frac{\text{amount of solution in litres}}{\text{amount of H}^+\text{ ions}}.$$]]
acidic = a solution whose [[$\text{pH}$]] is below 7
neutral = a solution whose [[$\text{pH}$]] is 7
basic = a solution whose [[$\text{pH}$]] is above 7

an acid = a substance capable of donating a hydrogen ion [[$\text{H}^+$]]
a base = a substance capable of receiving a hydrogen ion [[$\text{H}^+$]]

All solutions of acids are acidic, and all solutions of bases are basic.