I thought about this question as I first embarked on the online course Animal behaviour & welfare provided by the University of Edinburgh free of charge on Coursera, which I highly recommend if you're curious about this topic.
According to Wikipedia, "Animal welfare is the well-being of non-human animals. Formal standards of animal welfare vary between contexts, but are debated mostly by animal welfare groups, legislators, and academics [1][2]. Animal welfare science uses measures such as longevity, disease, immunosuppression, behavior, physiology, and reproduction, animal welfare science [3] although there is debate about which of these best indicate animal welfare."
"Respect for animal welfare is often based on the belief that non-human animals are sentient and that consideration should be given to their well-being or suffering, especially when they are under the care of humans [4]. These concerns can include how animals are slaughtered for food, how they are used in scientific research, how they are kept (as pets, in zoos, farms, circuses, etc.), and how human activities affect the welfare and survival of wild species."
A concept supporting animal welfare is the so-called 5 freedoms. This concept was first suggested by Professor Roger Bramwell in the 1960s. I created the infographic on the right to visualize them. Let me know your thoughts!
Grandin, Temple (2013). "Animals are not things: A view on animal welfare based on neurological complexity" (PDF). Trans-Scripts 3: An Interdisciplinary Online Journal in Humanities And Social Sciences at UC Irvine. UC Irvine. Archived from the original (PDF) on 19 August 2014. Retrieved 20 December 2013.
Hewson, C.J. (2003). "What is animal welfare? Common definitions and their practical consequences". The Canadian Veterinary Journal. 44 (6): 496–99. PMC 340178. PMID 12839246.
Broom, D. M. (1991). "Animal welfare: concepts and measurement". Journal of Animal Science. 69 (10): 4167–75. doi:10.2527/1991.69104167x. PMID 1778832.
"Draft of the Universal Declaration on Animal Welfare" (PDF). media.animalmatter.org. Archived from the original (PDF) on 2012-02-27