An estimate is the most knowledgeable statement you can make at a particular point in time regarding, cost/effort, schedule, staffing, risk, the ...ilities of the product or service.
Immature versus Mature Software Organizations 
Setting sensible goals for improving the software development processes requires understanding the difference between immature and mature organizations. In an immature organization, processes are generally improvised by practitioners and their management during the course of the project. Even if a process has been specified, it is not rigorously followed or enforced.
Immature organizations are reactionary with managers focused on solving immediate crises. Schedules and budgets are routinely exceeded because they are not based on realistic estimates. When hard deadlines are imposed, product functionality and quality are often compromised to meet the schedule.
In immature organizations, there is no objective basis for judging product quality or for solving product or process problems. The result is product quality is difficult to predict. Activities intended to enhance quality, such as reviews and testing, are often curtailed or eliminated when projects fall behind schedule.
In mature organizations possesses guide the organization-wide ability to manage development and maintenance processes. The process is accurately communicated to existing staff and new employees, and work activities are carried out according to the planned process. The processes mandated are usable and consistent with the way the work actually gets done. These defined processes are updated when necessary, and improvements are developed through controlled pilot-tests and/or cost benefit analyses. Roles and responsibilities within the defined process are clear throughout the project and across the organization.
Let's look at the landscape of maturity on estimating the work for those providing the funding for the work.
Projects are small, short, and while important to the customer, not likely critical to the success of the business in terms of cost and schedule.
- Informal or no estimating
- When there are estimates, they are manual, without any processes, and likely considered guesses
The result of this level of maturity is poor forecasting of the cost and schedule of the planned work. And surprise for those paying for the work.
Projects may be small, short, and possibly important. Some for of estimating, either from past experience or from decomposition of the planned work is used to make linear projects of future cost, schedule, and technical performance.
This past performance is usually not adjusted for the variances of the past, just and average. As well the linear average usually doesn't consider changes in the demand for work, technical differences in the works, and other uncertainties in the future for that work.
This is the Flaw of Averages approach to estimating. As well the effort needed to decompose the work into same sized chunks is the basis of all good estimating processes. In the Space and Defense business the 44 day rule is used to bound the duration of work. This answers the question how long are you willing to wait before you find out you're late? For us, the answer is no more than one accounting period. In practice, project status - physical percent complete is done every Thursday afternoon.
There is an estimating process, using recorded past performance and the statistical adjustments of that past performance. Reference Classes are used to model future performance from the past. Parametric estimates can be used with those reference classes or other estimating processes. Function Points is common in enterprise IT projects where interfaces to legacy systems, database topology, user interfaces, transactions are the basis of the business processes.
The notion that we've never done this before so how can we estimate, begs the question do you have the right development team? This is a past performance issues. Why hire a team that has no understanding of the problem and then ask then to estimate the cost of the solution? You wouldn't hire a plumber to install a water system if she hadn't done this before in some way.
4. Quantitatively Managed
Measures, Metrics, Architecture assessments - Design Structure Matrix is one we use - are used to construct a model of the future. External Databases referenced to compare internal estimates with external experiences.
There is an estimating organization that supports development, starting with the proposal and continuing through project close out. As well there is a risk management organization helping inform the estimates about possible undesirable outcomes in the future.
 Improving Estimate Maturity for More Successful Projects, SEER/Tracer Presentation, March 2010.
 Software Engineering Information Repository, Search Results on Estimates
 The Capability Maturity Model for Software