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.

1. One of the main causes of disease in poverty-stricken countries is the lack of access to clean water.
2. The reason I quit my course was my general lack of interest in the subjects we were covering.
3. I don't want to leave college lacking in skills. I need to study hard.

1. I would have got the job, but they said I lacked the necessary experience.
2. Since they took away team bonuses, there has been a real lack of motivation to reach our targets.
3. I don't think he'll be put forward for promotion. He lacks the necessary drive and ambition.
4. I had hoped to start my own business last year, but I lacked the investment I needed.