Separate

Turn your back on your partner and start moving forward. Caller will give you instructions as to how far to move. eg
- walk around the square
- around two (people), hook on end of line
- around one, hook on the end (or squeeze into the middle) of line

Split

Dancers move forward between two inactive dancers (splitting them). The inactive dancers move apart to let them through then move back together. You end up back-to-back with the couple that you split. Usually followed by "Separate" -- sometimes not explicitly called:
eg, Split 2, (separate) Around 1 to a line.