Reserve (accounting)

In financial accounting, "reserve" always has a credit balance and can refer to a part of shareholders' equity, a liability for estimated claims, or contra-asset for uncollectible accounts.
A reserve can appear in any part of shareholders' equity except for contributed or basic share capital. In nonprofit accounting, an "operating reserve" is the unrestricted cash on hand available to sustain an organization, and nonprofit boards usually specify a target of maintaining several months of operating cash or a percentage of their annual income, called an Operating Reserve Ratio. 
There are different types of reserves used in financial accounting like capital reserves, revenue reserves, statutory reserves, realized reserves, unrealized reserves.
Equity "reserves" are created from several possible sources:
Within the framework of capital increase by share premium a larger proportion of capital increase is placed into a capital reserve while the subscribed capital is increased by a minimum amount. This is because the initial losses are covered by the capital reserve. If capital increase was carried out fully or to a significant degree through the increase of subscribed capital, equity could easily fall to below the subscribed capital due to the losses.
Reserve is the profit achieved by a company where a certain amount of it is put back into the business which can help the business in their rainy days. The preceding sentence may give the unwary reader the sense that this item is an asset, a debit balance. This is false. A reserve is always a credit balance. Retained Earnings typically has a credit balance. If a firm wants to label part of Retained Earnings as a Reserve for Reinvestment, then that labeling does not harm, but neither does it do anything about making assets, liquid or otherwise, available for any day, rainy or otherwise.
Sometimes "reserve" is used in the sense of "provision". This is inconsistent with the terminology suggested by International Accounting Standards Board. For more information about provisions, see provision (accounting). The preceding is, indeed, correct IASB usage, but be aware in the U.S., under U.S. Generally Accepted Accounting Principles, "provision" refers to a debit balance, not a credit balance. "Provision" is a dangerous word to use in attempting to achieve clear communications in conversations with U.S. and IASB conversations. "Provision for Income Taxes" means expense in U.S. GAAP and liability in IASB vernacular.