palindrome, semordnilap, etc.

Palindromes are words that read the same backwards as they do forwards. For example: racecar, kayak.

It can be a sentence too, like
Madam, I'm Adam.
Never odd or even.
Marge lets Norah see Sharon’s telegram.
Able was I ere I saw Elba. (A fictive saying by Napoleon as he was being exiled to the island of Elba)

It can be long:
Doc, note: I dissent. A fast never prevents a fatness. I diet on cod.

Extraordinarily long:
Dennis, Nell, Edna, Leon, Nedra, Anita, Rolf, Nora, Alice, Carol, Leo, Jane, Reed, Dena, Dale, Basil, Rae, Penny, Lana, Dave, Denny, Lena, Ida, Bernadette, Ben, Ray, Lila, Nina, Jo, Ira, Mara, Sara, Mario, Jan, Ina, Lily, Arne, Bette, Dan, Reba, Diane, Lynn, Ed, Eva, Dana, Lynne, Pearl, Isabel, Ada, Ned, Dee, Rena, Joel, Lora, Cecil, Aaron, Flora, Tina, Arden, Noel, and Ellen sinned.

It can also be numerical, like those we use to keep dates. For example, these particular ones we just used for the past few days in September:

9-10-19 (September 10, 2019)
9-11-19
9-12-19
   .
   .
9-19-19

all mean the same whether read from left to right or right to left.

If you have picked November 11, 2011 (11-11-11) for your wedding date, there is really not much room for excuses for forgetting your anniversaries.

Legend has it that the famous Charles Bridge at Prague, Czech Republic was laid its first stone at 5:31 AM on 9th of July, 1357 by its superstitious namesake Holy Roman Emperor who believed such symmetrical, palindromic number (1357 9, 7 5:31) would bring extra strength to the bridge.

I always thought if these two big name companies, Adidas adding an "s", Toyota adding an "a" to the front of their names:

Adidas --> Sadidas
Toyota --> Atoyota

Then they both become palindromic, bringing additional fun to their marketing, maybe.

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In the case when reversing the letters of a word makes it a different one, it is called a semordnilap. (Notice semordnilap is the exact reverse spelling of the word palindromes) 

For example,

"desserts" spelled backwards makes "stressed"

"diaper" becomes "repaid"

“deliver no evil” becomes "live on reviled"  

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Look at the word SWIMS. Turn your head upside down. It still spells SWIMS! Such word is called an ambigram.

In many cases ambigrams are artistic designs that retain their meaning when viewed or interpreted from a different direction, perspective, or orientation. 

For example,

  

is a mirror image of the word wiki

 

conveys the idea of light being both a wave and a particle

 

is a mirror and rotational ambigram of an arithmetic operation illustrating the commutative property of 2+1+5=8

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Portmanteaus are the blending of two words into one to create a meaning for a singular concept. Words like brunch (breakfast lunch), motel (motor hotel) are among the most commonly used in our everyday vocabulary.

It can apply to people: Celebrity couple’s names such as Brangelina for Brad Pitt and Angelina Jolie, and Kimye for Kim Kardashian and Kanye West.

Places: New York City neighborhoods like Soho (South of Houston Street) and Tribeca (Triangle Below Canal Street); Mexico-California border cities such as Calexico and Mexicali; and a descriptive one like Hotlanta that says much about the temperature of Atlanta, Georgia. ‌‌

Internet: Webinar is a web seminar; malware is malicious software; emoticons are emotions expressed in icon form; and netiquette is a guide to how netizens (net citizens) should behave on the internet. 

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Can't end this without sharing some Chinese word play fun as well, like this one you might find on a tea cup:

可以清心也 (soothe my soul it certainly can)  

Expanding it into a matrix:

可以清心也 (soothe my soul it certainly can)
以清心也可 (can be used to soothe my soul)
清心也可以 (soothe my soul it might as well do)
心也可以清 (hence my soul be soothed)

也可以清心 (might soothe my soul as well)  

Notice each character takes turn leading a sentence, producing a slightly different but totally legitimate and meaningful sentence each time. And this is a symmetrical set: you can read not only left to right, row by row, but also top to bottom, column by column, and get meaningful sentences each time.

A bad-ass all-in-one palindrome, semordnilap, and ambigram in its own right, don't you agree?

As for the portmanteau: A good portion of Chinese characters are by design portmanteaus already. Take the character 清 in these sentences as an example: It is a combination of water氵 and deep blue 青, therefore means "clear (with water)", transcended to mean spiritual cleansing, soul soothing here.

Fun fun fun words!