Any relative, friend, or partner who has a significant personal relationship and provides assistance (physical, social, and/or psychological) to a person with a with serious health-related suffering due to severe illness.
These individuals may be primary or secondary caregivers, and may or may not reside with the person receiving care.

Different words are used in different countries: bystander (India), caregiver (USA and Canada), carer (UK, New Zealand, and Australia), etc.
The words may be prefixed with "family," "spousal," or "child" to distinguish between different care situations.

Hudson P and Payne S. Family Carers in Palliative Care: A guide for health and social care professionals. Oxford. Oxford University Press. 2009. p. 284.

A caregiver is a person who tends to the needs or concerns of a person with short- or long-term limitations due to illness, injury, or disability. The term “family caregiver” describes individuals who care for members of their family of origin, but also refers to those who care for their family of choice. This could be members of their congregation, neighbors, or close friends. Family caregivers play a significant role in health care, as they are often the main source of valuable information about the patient.

Johns Hopkins Medicine. Community Health. Internet. Accessed on September 6, 2021. Available at https://www.hopkinsmedicine.org/about/community_health/johns-hopkins-bayview/services/called_to_care/what_is_a_caregiver.html