I see a lot of women talking about this concept of “spark” in dating. Like

“I started talking to him and felt the spark and fell in love with him”

Or in a dead bedrooms example

“I love him but I don’t feel the spark anymore”

Or in an adultery example

“I was speaking to another guy and I felt the strongest spark I’ve felt in my life”

What is this spark? I’ve seen people here use this term as well, but without definition. What does it mean?