A skin is the look of a snake during gameplay. There are 66 different skins in Slither.io ; 9 of which you can use without sharing.
You can gain access to all skins if you share on Facebook/Twitter, but you can simply click them, and go back to the page without logging in, and then choose your skin.
Please note that you cannot automatically pick a skin until you share, and if you do not share, you will automatically get one of the default skins. If you have SlitherPlus.io, you will unlock many other skins where the original Slither.io doesn't have, though they are not official, and should not be regarded as such.
<tabber>Slither.io Skins=
Change Skin feature
Share on Twitter or Facebook to unlock skins
Types of Snake Skins
Accessed Normally (Default Skins)
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Normal Skins
Accessed-By-Sharing Skins
The USA Skin - Red, white, and blue with white stars
The Russian Skin - Red, indigo, and white
The German Skin - Orange, black, and red
The Italian Skin - Dark red, green, and white
The French Skin - Red, blue, and tinted white
The Polish Skin - Dark red and tinted white
The Rainbow Skin - It's a rainbow
The Swedish Skin - Indigo and tinted yellow
The Finnish Skin - Blue and tinted white
The Indonesian Skin - Tinted white and dark red
The Silver Skin - Tinted white
The 4chan Skin - Tinted violet and lime
The Brazilian Skin - Lime, tinted yellow, and blue with white
The Irish Skin - Light orange, dark green, and white
The Romanian or Colombian Skin - Yellow, red, and blue
The ArcadeGo skin - cyan and tinted yellow stars
The Block Skin - Dark violet and orange with block shaped antenna
The Yellow/Grey Skin - Bronze yellow and dark silver
The JackSepticEye Skin - Tinted green with one eye
The Lithuanian Skin - Yellow, red, and green
A black snake with yellow stripes
Taking the eighth extra skin, there are the same bulbed collars, but a different lavender skin.
A lavender-indigo snake dotted in white.
A dark-blue version of the previous skin
An orange snake with dark grey stripes
A snake in red, blue, yellow, green, and white
A red, pink, and white snake.
A blue, indigo, and white snake
The Kwebblekop Skin - Dark Orange with K antenna
The PewDiePie Skin - Cyan with brofist antenna
The Jelly Skin - Light green with an happy face similar to Jelly's logo
Third Batch Skins (UNOFFICIAL NAME) A red snake with grey and white stripes
A yellow snake with grey and white stripes
An orange snake with grey and white stripes
A cyan snake with grey and white stripes
An almost rainbow-skinned snake, with some grey and white stripes
A multi-colored snake with pink, purple, yellow, green, and many more colors
A red snake with blue stripes filled with white stars
A modification to the Yellow/Grey skin - it has blue collars with lights on them attached to its body
A cyan, orange, yellow, and grey snake
A yellow snake with lime stripes
An orange snake with dark gold stripes