Serdar (a variant of Sardar, a long-standing Indo-Aryan rank) was a military rank in the Ottoman Empire and a noble rank in Montenegro and Serbia. It means a head (ser) of place/land (dar). Serdars especially served at the borders of Ottoman Empire. They were responsible for security of lands. For example, Yakup Ağa who was the father of Barbaros from Yenice.
Serdar is also used as a noun (especially to give a nostalgic feeling) with its original meaning (i.e. commander-in-chief) in Turkish. For example, "Ordunun serdarı yiğit savaşçılarına saldırı emrini verdi" means "The commander-in-chief of the force ordered his brave fighters to attack."
The word has been adapted in English with its original or related spellings, i.e. serdar, sardar, sirdar, all meaning commander-in-chief of an army. However, these words are not used in everyday terminology.