A washout period can describe two scenarios:
a) the run-in period before a study begins during which researchers are waiting for a previous drug or intervention’s effects to wear off before they start a new one (to avoid confounding between the two interventions), or
b) the period between two different interventions when researchers are waiting for the participants’ bodies to normalize after the first intervention before beginning the next one.
An example of the latter might be a study in which people participate in a strictly defined sleep or exercise or diet routine for a couple of weeks and then, after a washout period of perhaps two weeks, participates in a different strictly defined sleep or exercise or diet routine to compare the effects of each one in each person (a type of self-controlled case series design or cross-over study).