"using namespace std;" is bad practice in C++

C++ is a very commonly used programming language which is used by many users who are active in "Competitive programming" and if they want to learn "Data structures and Algorithms" concepts and want to master in technical interviews for their dream job.

As C++ language is widely used nowadays but while writing its code you have to include some header files and a line "using namespace std;".But do you know including this particular line in a C++ code or a program is not considered a good practice?...