This topic has been talked about many many times and as yet I don't believe any consensus has been reached.
With LRM's being fixed? now and constrictions slowly being placed on poptarts, I still believe the question needs to be answered.
Within my company's dealing with other units, when the no cheese builds in a drop has been mentioned there has been some chatter as to what one really is.
I believe I have come up with an answer.
A cheese build will be any Mech that deals 40 or more points of damage with combined direct fire weapons in the concerned weapons overlapping max range bracket.
List of Weapons
Name..........Max Range....Damage
......................Bracket
S Laser..........90-180............3
S P Laser.......90-180...........3
M P Laser.....180-360...........5
M Laser.........270-540..........6
L P Laser......300-600.........10
L Laser..........450-900..........9
PPC..............540-1080........10
ER LLaser....675-1350.........9
ER PPC........810-1620........10
MG...............120-240..........0.8
AC10...........450-1350.........10
AC20............270-810..........20
LBX10..........540-1620 (only included for point blank use, Possibly excluded due to shotgun effect)
UAC5...........600-1800..........5
AC5.............620-1700..........5
GR...............660-1980.........15
AC2.............720-2160..........2
Examples
7xMLasers and 2xSLasers are not cheese, the Maxrange bracket for the MLasers is 270-540, the SLasers Maxrange bracket is 90-180, the two brackets do not overlap.
3xPPC are not cheese, 3xPPC and a Gauss rifle is.
4 Large Lasers are not cheese, 5 or more is.
An AC20 and 2xERPPC's are not a cheese build, the two weapon types have different working ranges, even tho if you close to short range the oponent can still Alpha you for 40 (technically this is borderline as the max range on the AC20 is 810 and 810 is where the ER PPC starts it's max range).
Note: the HBK-4P, if you replace the SLaser with a MLaser, becomes a cheese build, but shutting down after two alpha strikes at such a close range will get laughed at by many

Final say.
If you wish to flame on and call me a wimp because I am scared of fighting against Mechs that can alpha 40 or more damage, feel free. This is something that many people do want to have discussed as they hate being forced to play against Mechs that take no skill to pilot. Building a Mech that is effective at various ranges is what the Devs tried to encourage by putting in a range of both small and large maps and thats what this definition encourages. For those of you that wish to discuss the whole thing further please give feedback. I am not entirely happy with the way I have worded it and am still looking for a final version that isn't confusing.