California Rev. and Tax Code section 23101 defines what it means to do business in California, including a sales threshold for taxpayers not physically located in the state. A taxpayer is considered to be doing business in California "if it actively engages in any transaction for the purpose of financial or pecuniary gain or profit" where any one of a number of conditions are satisfied, including having $54,771 in real and tangible personal property (originally $50,000), $54,771 in payroll (originally $50,000), and $547,711 in sales (originally $500,000) in the state for taxable year 2016.
The original bright-line nexus threshold amounts have been adjusted for inflation every year since the code was first implemented, in 2011. To read the full list of conditions and to check the inflation-indexed thresholds for prior years, click here.