used in negative statement to describe a situation that has existed up to this point or up to the present time; "So far he hasn't called"; "the sun isn't up yet"
used after a superlative; "this is the best so far"; "the largest drug bust yet"
a streak of light in the sky at night that results when a meteoroid hits the earth's atmosphere and air friction causes the meteoroid to melt or vaporize or explode