LTI: Linear time invariant, linear time invariant
If you just look at this abbreviation, it is not easy to understand. In fact, this abbreviation is just a concept. It is easy to understand after adding a modified word system. It is equivalent to an example of understanding LTI.
LTI system refers to a linear time-invariant system, which means that this system, according to the input, has an output. The function between input and output is fixed, linear, and does not change over time. Of course, this is a very ideal system, just like a system without any friction. Or superconducting systems at ultra-low temperatures. Or systems on chips where rubbing doesn't generate heat.
In fact, any system, as long as it is running, cannot be an ideal system. There is always some resistance, either resistance, friction, or wind resistance; so the input and output functions of the system are not time-invariant, but time-varying. With the passage of time, the change of temperature, the strength of resistance will change more or less.
At the same time, having said that, the system can always be adjusted to a balanced state, so as to achieve a relative LTI level.