Lexis refers to word use. Lexicology is literally the study of words.
"Good", "sunny", "elated", "jolly" could all be said to be under a positive lexis, where as
"Death", "confrontation", "rain", "depressed" are all a negative lexis
It can often be used to infer people's demeanour
. Although people don't necessarily say how they feel, they're choice of words can be revealing.