Its defined as a condition where a person has low sperm count when he has fewer than 15 million sperms per of semen.(also known as oligospermia)

There are other forms of infertility which affect the sperm density to a lesser degree, as well as infertility caused by abnormal sperm mobility (making it harder for the sperm to swim to the egg) and sperm morphology (meaning that sperm are shaped abnormally and cannot fertilise the egg).

Causes
  Testicular problems
         undescended testicles
        surgery
        trauma
        cancer
        congenital defects.
        Blockage of testicular ducts sperm do not make it into the ejaculate.

  Hormonal problems
         Low testosterone
         drugs/medications
         congenital disorders
         tumours

  Ejaculation problems
         premature ejaculation (when ejaculation occurs too quickly)
         retrograde ejaculation (when semen is ejaculated into the bladder).

  Lifestyle
         Obesity
         Addictions(. Alcohol, Cigarette and Drugs)
         Unhealthy food habits

Treatment

  IUI
  Intracytoplasmic Sperm Injection (ICSI)
  Hormonal treatment
injectable hormones- Pergonal or Humagon,
  Antibiotic treatment:
  Surgery  
repair a blockage in sperm transport
  Intracytoplasmic Sperm Injection (ICSI) may be the best option to ensure egg fertilization.
  Magnetic assorting cell sorting(MACS) with(ICSI)
  Sperm DNA fragmentation
  Artificial donor sperm insemination