When you are sitting on an airplane and the person next to you is taller or fatter so parts of their body start to spread to your seay making you uncomfortable.
The obese woman's thigh was touching mine on the airplane. What a seat encroacher!
I had to put a pillow next to the seat divider because the man's elbow were in my space. I hoped when his elbow touched the pillow he would stop being a seat encroacher. At the very least, he'd be touching the pillow and not me.