According to ScienceDirect, inbreeding is defined as the probability of two alleles in an individual being identical by descent, and it is normally the result of mating related individuals. The second edition of Encyclopedia of Biodiversity defines inbreeding as mating between individuals that are related by ancestry and is more likely in populations that are, or have been, small. Inbreeding depression, on the other hand is defined by PNAS as the decrease in fitness with increased genome-wide homozygosity that occurs in the offspring of related parents.
In most species that are threatened, inbreeding remains a major concern and the reality is that the driver of evolution of mating systems in these species may be the avoidance of inbreeding. Yet, the individual inbreeding coefficients in populations is challenging and this still makes the overall effect on inbreeding to be unclear.
Inbreeding has a number of disadvantages especially a reduced efficiency in reduction which may be characterized by an increase in mortality rates, poor growth rates and a higher rate of hereditary abnormalities. Study to prove this has been carried out using cattle, sheep, horses, swine and other lab animals.
Science Direct "Genetic and brood stock management of Coho salmon." https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/inbreeding
PNAS "Inbreeding depression across the lifespan in a wild mammal population." https://www.pnas.org/doi/10.1073/pnas.1518046113#:~:text=Inbreeding%20depression%20is%20the%20decrease,the%20offspring%20of%20related%20parents.
Extension University of Missouri "Inbreeding: Its meaning, uses and effects on farm animals." https://extension.missouri.edu/publications/g2911#:~:text=The%20most%20obvious%20effects%20of,sheep%2C%20swine%20and%20laboratory%20animals.