A horizontal market is one that supplies goods to a variety of industries instead of just one. Therefore, horizontal market software is software that can be used by several different types of industries.
For example, word processing and spreadsheet programs are horizontal market applications because they can be used by many types of businesses and consumers. A business owner might use word processing software to type memos to his employees, while a student might use the same program to write a paper for a class.
Software developers that create programs for horizontal markets typically have a broad audience, but also face high levels of competition. The opposite of horizontal market software is vertical market software, which is only developed for a specific industry.