Define Mole and Avogadro constant?

Hint: We know that the mole (whose symbol is mol) is the unit of measurement for measuring the amount of a substance in the International System of Units (SI). A mole can be defined as the amount of substance that contains the same number of chemical entities.

Complete answer:
The mole is a measure or the base unit for the amount of substance present in the given sample. One mole is a number which is equal to \[6.022\times {{10}^{23}}\]particles, also known as the Avogadro’s constant. These particles can be any type of species, e.g. atoms, molecules, electrons, protons, neutrons etc.
Also, Micro-sized particles such as atoms, molecules, or ions cannot be seen or measured directly by physical balance neither can they be counted like everyday things. In the early nineteenth century, Amedeo Avogadro found that equal volumes of gases under the same conditions of temperature and pressure have equal numbers of molecules (Avogadro’s law). It was found that at pressure (Standard Temperature and Pressure, STP), they were all occupied of volume and their mass under the same conditions was their molecular mass expressed in grams. The Avogadro constant, named after scientist Amedeo Avogadro, is the number of constituent particles, usually molecules, atoms or ions that are contained in the amount of substance given by one mole.

Note:
Remember that the relationship of Avogadro’s number with mass, moles and volume must be mentioned properly and the fact that one mole of any substance (and not just gases) contains particles equal to Avogadro’s number. The particles may be atoms, molecules or ions.