The Nihang (also spelt as Nihung lit. "Crocodiles") or Akali (lit. "Immortals"), also known as Dal Khalsa, is an armed Sikh warrior order originating in the Indian subcontinent. Nihangs are believed to have originated either from Fateh Singh and the attire he wore or from the "Akal Sena" (lit. Army of the Immortal) started by Guru Hargobind. Early Sikh military history was dominated by the Nihang, known for their victories where they were heavily outnumbered. Traditionally known for their bravery and ruthlessness in the battlefield, the Nihang once formed the irregular guerrilla squads of the armed forces of the Sikh Empire, the Sikh Khalsa Army.

The word Nihang may come from the Persian word for a mythical sea creature (Persian: نهنگ). The term owes its origin to Mughal historians, who compared the ferocity of the Akāli with that of crocodiles. The meaning of Akali in Sikhism however, is the immortal army of Akāl (God). According to Harjinder Singh Dilgeer, tracing the term Nihang to the Persian word for a crocodile is a misinterpretation and instead it refers to a "fearless person". Dilgeer states that the term Nihang is also used in gurbani, where it refers to someone who is carefree.

The word Akāli means timeless or immortal. Literally, it means one who belongs to Akāl (beyond time). In other words, an Akāli is that person who is subject of none but God only. The term Akāli was first used during the time of Guru Gobind Singh. It became popular in the last decades of the eighteenth century. The term came to be associated with “commitment, fearlessness, boldness, struggle, and justice.”

According to Pashaura Singh and Louis E. Fenech, reiterating the work of Nabha (1999), there exists three main theories regarding the genesis of the Nihangs. These three theories are summarised below:

Dilgeer on the other hand narrates an entirely different theory of origination apart from the above three theories in his Sikh Reference Book. According to Dilgeer, the Nihangs originate from the period of Guru Gobind Singh, when during the Battle of Anandpur Sahib on 2 December 1703, the standard-bearer of the Sikhs, Bhai Man Singh, became injured and the pole of the flag broke, thus Guru Gobind Singh tore a strip of cloth from his blue-coloured under-turban (known as a keski) and tied it on the top part of his over-turban, creating what is known as a farla. Mimicking him, other Sikhs of the time, such as Uday Singh, Sahib Singh, Mohkam Singh, and Alam Singh emulated this manner of stylising one's turban with a blue-coloured strip of fabric. Guru Gobind Singh is then said to have mandated all Sikh leaders to follow this practice, so that the Sikh flag can never be lowered. Connecting to the theory regarding Fateh Singh, it is then believed Fateh Singh tried to also decorate his turban with a blue farla in this manner. This change to the Sikh uniform is said to have led to the arising of the Nihangs.

According to J. S. Grewal, the Nihangs originate from remnants of the Khalsa of the late 18th century who failed or refused to occupy any territory, and were not associated with government or administration.

It is claimed that the Nihangs arose as early as the period of the Akal Sena of Guru Hargobind, while other theories trace them to the period of Guru Gobind Singh, yet there are few Nihangs recorded in the history of the 18th and 19th centuries aside from Akali Naina Singh and Akali Phula Singh. Nihangs claim Baba Deep Singh as being a Nihang and also trace their legacy back to the bifurcation of the Dal Khalsa into to groups, the Buddha Dal and Taruna Dal, by Nawab Kapur Singh in the 18th century. Nihangs claim that their modern organisations sharing the same name can be traced back to these historical organisations of the Misl-era Sikhs. Akali Phula Singh was the overseer of the Akal Takht in Amritsar during his time and participated in the military conquests of the contemporary Sikh Empire. The deras of the Nihangs during Sikh-rule were kept independent from those of other Sikh groups. According to Dilgeer, the Nihangs reached the zenith of their influence during the late 18th and early 19th centuries. During Sikh-rule, the Nihangs maintained many Sikh places of worship. With the oncoming of the mantle of leadership of Akali Phula Singh, the Nihangs also became known by a new term, Akali. During the Sikh Empire and particularly after the death of Phula Singh, the Nihangs became increasingly addicted to cannabis and lost their influence. However, another change marked by the death of Phula Singh is Maharaja Ranjit Singh made an order of Sikhs, namely the sarbarahs and mahants, the custodians of Sikh shrines, with Nihangs assisting in this management structure.

During British rule, the Nihangs faced a decline, with there being fewer Nihangs, as they had lost the former prestige they held during the pre-colonial period. The system of managing Sikh shrines through sarbarahs, mahants, and Nihangs introduced during the Sikh Empire was continued by the British administeration. During the Singh Sabha Movement, the Sikh reformers may have adopted the unique markers of Nihangs in order to formulate and promote a separate Sikh identity, encapsulated as the Tat Khalsa. Aspects of the Nihang dress was adopted by other Sikhs, as it was seen as being "quintessentially" Sikh, alongside the martial aspects of the Nihangs, which was promoted by the British administrators as being ideal Sikh traits. The Akalis of the 20th century cannot be traced back to the historical Nihangs. Since 1920, the term Akali is also used synonymously to refer to members of a Sikh political party called the Shiromani Akali Dal.