Absence means the state of being away from someone or something, or to not be present.
Ex: No absences are allowed during this course. You must be here for every single lecture in order to pass the final exam.
Ex: Our swimming team really missed you during your absence. We could not find a replacement swimmer and had to forfeit the gala.
The common saying absence makes the heart grow fonder means that you tend to appreciate people more when you have gone without them for a period of time.
Ex: My boyfriend and I haven't seen each other for three months while he has been travelling for work, but you know what they say—absence makes the heart grow fonder!
Ex: My father passed away five years ago, but his absence has only made my heart grow fonder! I really admire the way he lived his life and raised my brother and I.
To lack means to not have something that you need or not have enough of something.
Ex: We wanted to hire that friendly woman for the position, but she lacked the necessary experience in a banking position.
Ex: That woman would be a wonderful speaker if she didn't lack self¬confidence. She has great ideas, but she is so shy you can hardly hear her when she speaks.
The phrase, or lack thereof, is often used to express that the absence or shortage of something is actually the true case. It is used to express sarcasm.
Ex: My family's great wealth, or lack thereof, is what has led us to live in this tiny flat in inner¬London for most of our lives.
Ex: My boss's charm, or lack thereof, has caused ten of our best employees to hand in their notice in the past two months. I might be next!