A partial fill means that you have asked your broker to buy or sell stock, but the broker can't buy or sell as much as you would like, and a portion of the order remains unfulfilled. Every time you trade stocks, you're charged a commission even if it's partially fulfilled. To refrain from partial fills, you will need to put restrictions on your orders.
You'll be charged a commission by your brokerage even if your limit order is partially fulfilled, possibly spanning multiple days and stacking your commissions.
Partial Fills and Limit Orders
Partial fills are especially common with limit orders, where you instruct your broker to buy or sell shares if the price hits a certain point. People use these types of orders to make sure they can get stock when it's cheap or sell it when it's pricy.
Naturally, if you're buying stock, you'll pay the cost of how much stock you've managed to acquire. If you're selling stock, you'll receive the amount for which it sold. However, as with other transactions, you'll also be charged a commission by your brokerage.
Commissions for Partial Fills
Generally, you'll be charged a commission for every day that part of your order is filled. That means that if it takes multiple transactions in one day to buy or sell the stock you want, you'll be charged a single commission, but if it takes multiple days to fill your order in full, then your commission charges can start to stack up. It's usually a good idea to understand your broker's policies on fees for any trade you're considering in order to make sure it's still a good deal, so check your broker's website or give him a call.
When you place a limit order, you usually have multiple options for how you want it filled. You can specify that it's good until the end of a day, in which case it will be partially or fully filled if the price condition is met, and you will be charged a single commission for whatever trades were done.
You can also specify that the order is good for a longer period of time or is good until cancelled. In that case, if the price crosses the limit on multiple days, you may end up buying or selling more stock, but you will also incur more commissions.
Avoiding Partial Fill
Some brokerages will also allow you to specify that you want your order filled in full or not at all, avoiding the risk of a partial fill. This may be worth it if you're worried about incurring commission costs for small transactions. This is sometimes called a "fill or kill" order. In some cases, you may be able to specify a minimum number of shares for the transaction. You can also sometimes put additional restrictions on orders, like that they only be executed at the beginning or end of a trading day.