Several states in the Zhou dynasty, which had lasted for many generations, were captured during the Warring States period. Eventually, amongst the self-styled kings of the independent states, one from the strongest state overcame the rest in 221 BC.
This was the Emperor of the state of Qin. He had exceptional ability as a leader, supported b a strong army and was able to manipulate the other six states. This Emperor, Ying Zheng called himself First Emperor and that is why his period, the Qin Dynasty, is known as the first Imperial (emperor) period. He didn't rule long, but managed to unify China as one political entity amongst other important achievements.
The Qin dynasty was called the first imperial dynasty of China because it was the first empire that unified China. It was the first unified, multi-national and power centralized dynasty in the Chinese history. Before then, China was ruled by dynasty state. The unification of China in 221 BC under the first Emperor Qin Shi Huangdi marked the beginning of the Imperial China. Qin dynasty gave China its name.
Although the dynasty lasted only for 15 years, the dynasty held an important role in the Chinese history, it left important marks on Chinese culture and it had a great influence on the following dynasties.