I find the term "line of sight" to be a bit unclear. According to the Sage Advice Compendium, D&D 5e uses the English meaning of "line of sight":
Question: The frightened condition says “while the source of its fear is within line of sight.” Does that mean you have disadvantage on attack rolls and ability checks even if the source is invisible but you have a clear line to its space?
Answer: No. If you can't see something, it’s not within your line of sight. Speaking of "line of sight", the game uses the English meaning of the term, which has no special meaning in the rules.
Looking through various English dictionaries, I found the following definitions:
- Merriam-Webster: A line from an observer's eye to a distant point
- Dictionary.com: Also called line of sighting. An imaginary straight line running through the aligned sights of a firearm, surveying equipment, etc.
- Google Dictionary: a straight line along which an observer has unobstructed vision.
According to these English definitions of "line of sight", it is possible to have a line of sight to an invisible object, so long as the path between the observer's eye and the invisible object is unobstructed. This directly conflicts with D&D 5e's definition, according to which it is not possible to have a line of sight to an invisible object.
What English definition of "line of sight" is 5e using, then?