Anything between freshwater and marine water is considered brackish water. You will have to buy a refractometer to measure the salt level. Depending on the type of fish you want to keep, you will either have to keep your water constant at low-end, moderate, or high-end brackish water.
Best practice is to pre-mix your water before adding it to your tank. People use a trash bin, fill it with water, add some salt, and let it mix with a powerhead and heater for 2-3 hours before adding it to the main tank. Easy to measure and match the salt level of the main tank.