Sorties - what do they really mean?

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Well, I can't hide the face since last mylot's site upgrade, the home page became different and more organized. Yes, I do love how it looks like, but I cannot understand what sortie means. LOL. I am sorry for my innocence but is there anyone out there who can tell me what this means? I try to looked in the dictionary and it seems like this word is not yet includeded in it. No reason in particular. I would just like to feed my curiosity about this. Thanks a lot.