Marketweight

You run a mutual fund. Your portfolio's performance is measured against the S&P 500. You're supposed to have about 2% yield. And you need to do a percent or three better than the S&P 500 each year, compounded, to warrant the fees you charge, which are much higher than those of an index fund.

If you were trying to be exactly equal to the S&P 500, you'd note that 14.7% of it, today, comprises telecom stocks. But you're bullish on telecom from the prices where they're generally trading today, so you want to be overweight that sector. So in your portfolio, maybe you hold 23.5% telecom stocks. If you were just marketweight, then you'd have 14.7% of your portfolio represented in that sector.

Marketweights usually apply to sectors, but sometimes you get enormous market cap single stocks, like Apple or Amazon or Google or Facebook, and you have to then take a position to be over-or-underweight those stocks...or just have them represent in your portfolio whatever they rep in whatever index against which your portfolio's performance is being measured. Easy.