What is the definition of an invalid / disabled passenger?

What is the definition of an invalid / disabled passenger?

The term ’invalid or disabled passenger’ is used by airlines to denote those passengers who due to sickness or disability may require special assistance or consideration prior to, during, or immediately following an air journey. Invalid passengers may be of 3 types -
(a) those who are travelling to obtain treatment for their ailment;
(b) those with terminal illness who wish to travel home to die - most airlines will consider such cases sympathetically on humanitarian grounds;
(c) those who are travelling for business / pleasure, and whose disability or illness is incidental to their journey.
Medical problems are commonly encountered with passengers in the first group, firstly, as they are usually seriously ill and require assistance / medications during the flight, and secondly because the flight environment is ’hostile’ and may significantly affect their condition.