DNA complementarity against reverse complementarity
I agree that the definitions can be somewhat confusing when first encountered.
Each nucleotide has a complement A-T, C-G. But the DNA strand are reverse-complementary because when aligned from 5'-&gt;3' they are not (necessarily) complementary.
For example:
DNA strand 5'-ATCCGG-3'
complement 3'-TAGGCC-5'
reverse    5'-CCGGAT-3'
Since we want to write all sequences in the same direction we must call the strand reverse-complementary