Polar and nonpolar covalent bonding refer to different degrees of electron sharing between atoms in a covalent bond.
In a polar covalent bond, the electrons are shared unequally between the atoms involved. One atom has a higher electronegativity (ability to attract electrons) than the other, resulting in an uneven distribution of electron density. This creates partial positive and partial negative charges on the atoms, forming a dipole. The greater the electronegativity difference between the atoms, the more polar the bond.
In a nonpolar covalent bond, the electrons are shared equally between the atoms. The atoms have similar or identical electronegativities, resulting in an even distribution of electron density and no significant charge separation.
The reason polar and nonpolar covalent bonding may be used interchangeably is that they represent the two extremes of a continuum of electron sharing. In reality, most covalent bonds lie somewhere between purely polar and purely nonpolar, with varying degrees of electron density distribution.
When a compound has both types of bonds within it, it is more accurate to describe it as having polar and nonpolar covalent bonds or a mixture of polar and nonpolar character. This recognizes that different bonds within the compound exhibit varying degrees of electron sharing. Using both terms acknowledges the complexity and diversity of the bonding nature within the compound.