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I am seeing conflicting answers on the net

e.g.

A carbon atom is chiral if it has four different items bonded to it at the same position. (Wiki)

compared to

Stereocenter (chiral center): An atom with three or more different attachments (Chem.Ucla)

Why does one say 3 or more, and one say 4?

Also, there is a double bonded carbon atom here that is said to be a stereocentre, which I cannot understand. Isn't the double bond counted as two of the "same" attachments (so not even satisfying the 3 different groups criteria). How is that still a chiral centre?

enter image description here

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    $\begingroup$ It looks like the difference is that the UCLA definition doesn't include a hydrogen atom as an attachment, so 3 non-hydrogen attachments qualifies if the fourth "attachment" is a hydrogen. $\endgroup$
    – airhuff
    Jul 2 '17 at 5:13
  • $\begingroup$ How about the middle picture in the Ucla Link. There is a double bonded carbon, and it calls this a stereocentre/chiral centre. So there is no 4th attachment right? Also, isn't the double bond counted as two of the "same" attachments (so not even satisfying the 3 different groups criteria). How is that still a chiral centre? Thank you! $\endgroup$
    – K-Feldspar
    Jul 2 '17 at 5:15
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    $\begingroup$ That's okay. Thank you anyway :) I thought I had chirality down pat until I saw this ... which is going to lead to my next question of why cis/trans isomers fall under the category of diastereoisomers, but I will save that for a separate question after this one has been answered. $\endgroup$
    – K-Feldspar
    Jul 2 '17 at 5:22
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    $\begingroup$ That image doesn't make sense. In an alkene with E/Z isomerism the unit that leads to stereoisomerism is the alkene, not one particular carbon of the alkene. None of the alkene carbons are chiral centres. Clearly it seems that UCLA have their own idea of what a stereocentre is, but it doesn't make sense to me for the stated reasons above. $\endgroup$
    – orthocresol ♦
    Jul 2 '17 at 7:09
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    $\begingroup$ Regarding the diastereomer thing see chemistry.stackexchange.com/a/68709/16683 the definition of diastereomer is simply a stereoisomer that is not an enantiomer, which E/Z isomers fall into. $\endgroup$
    – orthocresol ♦
    Jul 2 '17 at 7:10
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The answer is that all chiral centers are stereocenters, but not all stereocenters are chiral centers thus the words are not interchangeable.

A chiral center requires four or more different molecular groups attached to a single center atom (asymetrical). In the case of pnictogens (nitrogen, phosphorus, etc...) or chalcogens (sulfer, selenium, etc...) an electron pair can count as one of the molecular groups.

An alkene is a stereocenter that requires 3 or 4 different molecular groups such that no two common groups are on the same side of the bond (must be able to determine an E/Z configuration). In the case of the blue stereocenter, the methyl groups are on the same side and thus switching their position would create an identical compound meaning the alkene is not a stereocenter. For the red center switching the position of the bromine and methyl groups would create the Z-isomer from the E-isomer thus is a stereocenter, but not a chiral center. If however, the hydrogen of the blue stereogroup were to switch positions with a methyl group then the blue alkene would be a stereocenter with three distinct molecular groups.

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