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Definition of DL. 1  DL is short for down-low, which basically means to keep things under-wraps, to keep a secret or lay low. 2  I'll tell you this, but, you gotta promise to keep it on the DL. D.L. means down low. it means that something is not known to all. if you keep something on the D.L... 2  you don't share it with others. 3  I don't want my Dad to find out, so keep it on the D.L. 4  She like me but keep that on the DL.
The #define directive takes two forms: defining a constant and creating a macro. When defining a constant, you may optionally elect not to provide a value for that constant.In this case, the token will be replaced with blank text, but will be defined for the purposes of #ifdef and ifndef.f a value is provided, the given token will be replaced literally with the remainder of the text on the line. You should be careful when using #define in this way; see this article on the c preprocessor for a list of gotchas.