The term 'SECOND OF A SERIES' can be represented by the crossword solution BETA, which is a four-letter word. BETA is a term used to describe the second version of a product or service that is released for testing purposes. It is usually released after an initial Alpha version and before the final version. BETA versions are typically made available to a limited number of users in order to identify any remaining issues before the product or service is released to the public.