Semantic dyslexia is, as the name suggests, a subtype of the group of cognitive disorder

Cognitive disorder

Most common mental disorders affect cognitive functions, mainly memory processing, perception and problem solving. The most direct cognitive disorders are amnesia, dementia and delirium. Others include anxiety disorders such as phobias, panic disorders, obsessive–compulsive disorder, generalized...

s known as Alexia (acquired dyslexia). Those who suffer from semantic dyslexia are unable to properly attach words to their meanings in reading and/or speech. When confronted with the word "diamond

Diamond

In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

", they may understand it as "sapphire

Sapphire

Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...

", "shiny" or "diamonds"; when asking for a bus

Bus

A bus is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers. The most common type of bus is the single-decker bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are...

 ticket

Ticket (admission)

A ticket is a voucher that indicates that one has paid for admission to an event or establishment such as a theatre, movie theater, amusement park, zoo, museum, concert, or other attraction, or permission to travel on a vehicle such as an airliner, train, bus, or boat, typically because one has...

, they may ask for some paper or simply "a thing".

Semantic dementia (SD) is a degenerative disease characterized by atrophy of anterior temporal regions (the primary auditory cortex; process auditory information) and progressive loss of semantic memory. SD patients often present with surface dyslexia, a relatively selective impairment in reading low-frequency words with exceptional or atypical spelling-to-sound correspondences.