1) to fall in standard or quality (slip-up)

2) (to make) a leap or fall in which one turns over completely before landing on one's feet (somersault)

3) inflation with a fall in productivity (stagflation)

4) the regular rise and fall in the level of the sea, caused by the attraction of the moon; a change in amount (of public opinion, feeling etc); to help a person to get through or survive (a period of difficulty etc) (tide)

5) (of surfaces) to rise and fall in gentle slopes (undulate)