It has changed from when the terms were first introduced. Smokers/growers have perverted the terms for their own purposes.
Today, in our world, sativa refers to the very thin leafed varieties. Typically from the south east region of what would be referred to as Asia. These include Cambodia, Thailand laos north west china, etc. often having a different sexual reproductive system known as x to autosome which is a dosage tipping point system.This system is one whereby there are only females in the population, but if a female inherits enough male modifiers in her autosome, she will produce male flowers, sometimes exclusively if sufficient modifiers are present. Often exhibiting intersexed flowers. Creating effects on the user of a high, cerebral type.
Indicas on the other hand tend to be from Pakistan, India, Afghanistan and surrounding areas. These plants often but not exclusively have fatter leaves. Their sexual reproductive system is Active Y. Which means they are either male or female. Effects of usage tend to include a physical effect, feelings of sinking into surfaces, becoming too heavy to move, numbing effects. Mental effects rather than being stimulating, are also often numbing and emptying of thought, sleep inducing or stress relieving. A sedative of sorts.
Hybrids can exhibit any combinations of the above depending on what they were bred for.