Princeton's WordNet

1. meter, metre, m(noun)

   the basic unit of length adopted under the Systeme International d'Unites (approximately 1.094 yards)

   Synonyms:
   time, metre, one thousand, meter, beat, yard, cadence, m, molarity, mebibyte, measure, thou, megabyte, grand, molar concentration, thousand, chiliad

   Antonyms:
   ordinal

2. molarity, molar concentration, M(noun)

   concentration measured by the number of moles of solute per liter of solution

   Synonyms:
   megabyte, chiliad, metre, molarity, thousand, molar concentration, one thousand, mebibyte, thou, yard, meter, grand

   Antonyms:
   ordinal

3. thousand, one thousand, 1000, M, K, chiliad, G, grand, thou, yard(noun)

   the cardinal number that is the product of 10 and 100

   Synonyms:
   railyard, megabyte, special K, green, guanine, gigabyte, super C, kibibyte, potassium, thousand, mebibyte, g-force, metre, cat valium, universal gravitational constant, atomic number 19, gibibyte, railway yard, kB, curtilage, grounds, yard, grand, super acid, kelvin, gramme, pace, kilobyte, gee, chiliad, cubic yard, molar concentration, deoxyguanosine monophosphate, molarity, meter, honey oil, constant of gravitation, gravitational constant, one thousand, gram, grand piano, thou, gm, jet

   Antonyms:
   ordinal

4. megabyte, M, MB(noun)

   a unit of information equal to 1000 kilobytes or 10^6 (1,000,000) bytes

   Synonyms:
   megabyte, chiliad, metre, molarity, thousand, molar concentration, one thousand, mebibyte, thou, megabit, yard, meter, grand

   Antonyms:
   ordinal

5. megabyte, mebibyte, M, MB, MiB(noun)

   a unit of information equal to 1024 kibibytes or 2^20 (1,048,576) bytes

   Synonyms:
   chiliad, metre, molarity, thousand, molar concentration, one thousand, mebibyte, megabyte, megabit, thou, yard, meter, grand

   Antonyms:
   ordinal

6. M, m(adj)

   the 13th letter of the Roman alphabet

   Synonyms:
   megabyte, chiliad, metre, molarity, thousand, molar concentration, one thousand, mebibyte, thou, yard, meter, grand

   Antonyms:
   ordinal

7. thousand, one thousand, 1000, m, k(adj)

   denoting a quantity consisting of 1,000 items or units

   Synonyms:
   m, one thousand, thousand, k

   Antonyms:
   ordinal