"The half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one-half (50%). The symbol for half-life is t½."
https://www.drugs.com/article/drug-half-life.html
"Drug Half-life (t1/2) is defined as the amount of time required for the drug concentration to be reduced to exactly half its initial concentration or amount in the blood. The half-life is the time required for half of the drug to be metabolized and eliminated from the body. The half-life of the drug is useful in determining the dosing frequency"
https://www.medicinenet.com/what_does_half_life_mean_in_drugs/article.htm
"The elimination half-life of a drug is a pharmacokinetic parameter that is defined as the time it takes for the concentration of the drug in the plasma or the total amount in the body to be reduced by 50%. In other words, after one half-life, the concentration of the drug in the body will be half of the starting dose."
https://www.news-medical.net/health/What-is-the-Half-Life-of-a-Drug.aspx