NameError: name 'sess' is not defined

I am running python program under tensorflow. When I input sess.run(),the command line prompt me that NameError: name 'sess' is not defined

print(sess.run(W_conv1))
Traceback (most recent call last):

  File "<ipython-input-17-cf7d3892efbb>", line 1, in <module>
    print(sess.run(W_conv1))

NameError: name 'sess' is not defined