Today's names: Alastair, Alistair and Alasdair
Pronunciation: AL-ah-stayr, AL-uh-stayr, AL-us-dare
Potential nicknames: Al, Alic, Alix, Dair
Origin: From the Greek name Alexandros (Alexander), meaning "man's defender." Alastair and Alistair are anglicised forms of Alasdair, the Scottish Gaelic form of Alexander. Other variants include: Alasdhair, Alasteir, Alastar, Alaster, Alastor, Alaisdair, Alaistair, Alaister, Aleister, Alester, Alistair, Alistar, Alister, Allaistar, Allaster, Allastir, Allistair, Allister, Allistir, Allysdair, Allystair, Allyster, Alysdair, Alysdare, Alystair, Alyster, Allastair, and Aldair.
Popularity: In 2010 there were 15 baby boys named Alasdair, 25 Alastair, 6 Alaster, 81 Alistair, 5 Alistar, 28 Allister, and 29 Alister. In 2011 there were 108 Alistair, 57 Alister, 33 Allister, 27 Alastair and 23 Alasdair. Jump to 2015 and 194 boys were given the spelling Alistair, 36 named Alasdair, 47 Alastair, 28 Aleister, 81 Alister, 46 Allister, and 8 Allistair. The spelling Alastar was only used 5 times in 2011 and 5 times in 2013. The spelling Alaster seems to have ceased usage since 2013, while Alastor has just begun being used since 2011.
Fun fact: (1) There are several famous/well-known people with this name, one of which being Rod Stewart's son, and several fictional characters. (2) Three medieval Scottish kings were named Alasdair. (3) The surname McAllister/Macallister means "son of Alistair."