1. without or failing to have (something)
Professional
Social
lack (of) motivation/interest/ambition
The verb 'to lack' means to be without or to be deficient in something. When something is lacking there is not enough of something that is needed or wanted or necessary. The noun 'lack' is a state of not having enough or being without something. The phrase 'a lack of' means that something is missing or not present in enough quantities.