Unfortunately, this depends a lot on context. It could be the omission of a noun if used literally, but it almost never is. The sentence "Take a second." would normally be the omission of a prepositional phrase. The idiom is normally "take a second to..." It usually references thinking or actively sensing, i.e. "take a second to think through the problem before using brute force." Other phrases like this are "take a minute" or "take a moment" and have the same meaning. It does suggest something momentary though, as opposed to the related "take a while."
Interestingly, if someone says that something will take a second, you can expect them to be done shortly, but not literally in a second. If somebody will take a while, do not expect them to be done soon. These declarative (stating facts about the world) meanings carry through to the imperative (requesting or ordering that something be done) meanings as well.
There is a cockney rhyming slang for 'take a butchers' which means 'take a look' which is the only reason I can think of for confusing these two sentences otherwise they're entirely separate.
– icc97 – 2015-10-13T10:55:07.073@icc97 specifically its "butcher's hook" == "look" so "have a butcher's" would fit as well. – Criggie – 2015-10-13T21:35:59.150
1The confusion is caused by the ambiguity in the word "second". Did you know that the etymology is from the original Greek where the hour was divided into sixty "minute" parts, and each of these parts were subdivided into sixty smaller parts again, i.e. a "second" subdivision into smaller parts. – Mr Lister – 2015-10-14T15:25:21.977
Did I say Greek? Sorry, Latin. – Mr Lister – 2015-10-15T08:05:12.157