To follow up on this, Delta has the clearest standards I have seen, even more precise than the CDC. I've cut/pasted relevant portions below, adding boldface:

[Y]ou commit that you and, to your knowledge, those in your itinerary have not been diagnosed with COVID-19 within the past 10 days.You also commit that you and, to your knowledge, those in your itinerary have not experienced onset of any primary COVID-19 symptoms within the past 10 days:

• A fever of 100.4 degrees F or higher
• Feeling feverish
• New persistent coughing
• Shortness of breath or breathing difficulties
• Loss of taste or smell