Likelihood
\ ˈlaɪkliˌhʊd \
Likelihood is a statistical measure of how likely the observed data is under the fitted model.
When fitting a statistical model to a set of data, a common technique is to adjust the parameters of the model to maximize the likelihood of the observed data.
The natural logarithm of the likelihood function is known as the log-likelihood. Maximising the likelihood is equivalent to maximising the log-likelihood, so we are free use whichever is easier to work with.