Lasik/Lasek - what constitutes a 'significant' change in prescription?

Hi all,

I am 25 and have had glasses for about 8 years (only worn daily for a few years). I am considering laser surgery and notice in the ‘am i suitable’ type info that your prescrip shouldn’t have changed much in the last two years. Most websites are so commercial they don’t really tell you - how much is a big change? e.g. I went from -2.00 and -1.75 to -2.25 and -2.00 in about a year. Is that a lot? Also, what does ‘cylinder’ refer to on a prescription? Mine is -0.50, whatever that means. Cheers.