Русско-англо-русский юридический словарь

Business income is any income that is realized as a result of business activity. (Investopedia)

The meaning of to write something off as a business expense is deducting an ordinary and necessary expense for your business. For example, paying for advertising goods and services of businesses is a very common business expense. This expense is deducted from the business income and that reduces the amount of tax owned on the business income. (Quora)