I'm struggling to read the place of birth of Elizabeth Jones (Head of household schedule 167) in this image from the 1871 census of Merthyr Tydfil, Glamorgan, Wales.
Here is a larger image of the problem word.
Identifying a Welsh place-name does take a little extra effort sometimes. This particular problem is compounded by the fact that the county is not identified. To sort out a Welsh place-name the following steps may help. Some steps may help in your specific situation while others may only work in other cases.
Step 1: Check a gazetteer There are several published gazetteers for Wales but none are anything close to comprehensive. The Melville Richards Archive is the best source at http://www.e-gymraeg.org/enwaulleoedd/amr/cronfa_en.aspx where you should be sure to read the user guide for search tips.
Step 2: Find the person in every census during their life time. Census records may provide different clues from one census to the next. Until you have found your ancestor in every available census don't give up. Their birthplace may be identified more clearly in another census.
Step 3: Search records that list place-names within the parish:
Step 4: Examine maps The Ordnance Survey six inch to one mile scale maps will identify most farm names and places in the area where the family lived. Browsing the place-names in the area where a family lived may help solve a place-name problem. County record offices and the National Library of Wales should have the detailed maps needed. The Landranger and Pathfinder series of maps can be purchased and may be useful but are modern publications and not nearly as detailed as the the early editions of the six inch scale maps.
Step 5: Contact the local history society Local and family history societies often have members who are very familiar with the area. They may already have solved the place-name problem during some of their research.
Step 6: Learn more about the family in question. Find out as much about the family as possible. Don't depend only on the census. Find the marriage. Order birth certificates to verify information about the parents.
I believe the name starts with either "Cwm" or "Crwm". There is a bit of a stroke after the ending loop of the "C" which could be interpreted as an "r". Many Welsh place names start with "Cwm", and a few start with "Crwm", so either is plausible.
I'm pretty sure the name ends with "th". The "t" is a common form which makes a half-crossing stroke without lifting the pen by continuing from the bottom of the ascender and curving into a short horizontal on the right side of the ascender (which here is continued into the "h"). There are similar examples elsewhere on the same census sheet.
The middle letters are a muddle to me. My best guesses might be "seca" or "reca", but really there many unsure interpretations for these mysterious strokes.
The best way to resolve this would be to look at a few comprehensive Welsh gazetteers or place-name lists for names beginning with Cwm or Crwm. Unfortunately I can't recommend any, and did not find any in a limited web search. The best I turned up was this Ordnance Survey guide. The only name I encountered that might fit with my guesses was "Cwmcrwth".
You are not the only one who had trouble reading it ;) The person transcribing the 1871 census for Family Search also couldn't decipher it and they left it blank.
As @Jeremy mentioned, I think that this begins with Cwm... My best guess is "Cwmreash" or "Cwmrecash" with the second last consonant being a strong "s". Trying to say this in your head with a Welsh accent, might help you to imagine how an enumerator might misspell the word. Perhaps an "sh" should really be a "th" or "dd"? Perhaps the middle syllable got slurred? Roll it around on your tongue.
Also, could this be not a lost town, but a region? Glamorganshire is mentioned numerous times, so is Monmouthshire... Could it be a lazy-man's version of Carmarthenshire or Carmarthen?
I feel pretty confident that the prefix of the place is "Cwm" - this is a common prefix in Welsh - it means "valley", and it pronounced "koom" (rhyming with "room").
I agree with the other answers, but found one other (far-fetched) possibility. Google Maps has a "Cwmcathan Isaf" about 30 miles away from Aberdare. I think that the place written might be "Cwmcath", short for "Cwmcathan".