Question

define covalent bond

Answer 1

http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bond.html

Answer 2

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding.

Answer 3

In a nutshell:

A region of relatively high electron density between nuclei which arises at least partly from sharing of electrons and gives rise to an attractive force and characteristic internuclear distance.
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1101

Answer 4

Agree with @jagr2713
And if you need an example, see this drawing and description below.

Hydrogen has one electron in its valence shell and chlorine has seven... Both share one electron from their side and we have a shared pair between them formed due to covalent bonding. :-)
I hope it to be helpful. :-)