This term has more than one meaning depending on the context in which it is used:
In competition law, a letter stating that the European Commission (www.practicallaw.com/A35081) intends to close its file and take no further action in relation to an agreement which has been notified to it.
In banking terms, a written assurance often provided by a parent company in respect of its subsidiary's financial obligations to a lender. It is usually used where the parent company is unable or unwilling to give a guarantee but wishes to give some comfort to the lender in respect of the subsidiary's ability to perform its obligations. It is not usually intended to be legally binding but it may give rise to a legally binding obligation depending on the wording. Therefore, care is needed when drafting a comfort letter.
In equity offerings, a letter from the issuer’s accountants delivered to the underwriters (www.practicallaw.com/A37148) as part of the due diligence (www.practicallaw.com/A34925) process to provide, amongst other things, independent assurance that no serious adverse changes have occurred in the financial condition of the issuer during the period since its last audited financial statements up to closing.